How about the judgement of an average reasonable sailor represented by the members of the protest committee?

What kind of boats are we talking about?

In Harbor 20s, 80 meters = 240 feet = 12 boatlengths.

In 100 foot maxis, 240 feet = 2.4 boatlengths, and a lot of complexity to turn. 

Harbor 20s.  They're going 20 meters in 10 seconds in 10 kts of breeze.  That's 4 kts.  The maxi would be doing 10 kts upwind in 10 kts of breeze.

Novices in a starboard tack boat really like rule 10 though.  That can be an added element to the scenario.

Case 123 Abstract 
If, at the moment it would be clear to a competent, but not expert, sailor at the
helm of a starboard-tack boat that there is substantial risk of contact with a port-tack
boat, but there is time for the starboard-tack boat to change course
sufficiently to avoid the contact, she will break rule 14(a) if contact occurs.

Competent, but not expert is the normal standard used.

John Allen alluded to this in his comment.

I agree to the phrase " the moment it would be clear to a competent, but not expert, sailor at the
helm " as the key variable.

In a close quarter situation, however, there are many other variables like the boat- size, shape, rudder effectiveness etc, then the environment-wind speed, steady or gusts, waves, swell, visibility etc and last but not the least the level of competition- it sets a bar for the competence of the crew expected.

The answer lies in interpretation of the situation based on the evidence by the parties and the jury's application of their perception of the environment (this may not be written anywhere but practically always boils down to the perception of each member of the panel. The perception is dependent on so many variables, that itself can be a separate topic of discussion)

Why does green not hail "starboard" to red? 

40 seconds is an eternity to sailboats going 4kts. In cars and on bikes, we make split-second decisions all the time.
Also, 10kts is the kind of breeze where turning is easier, boat speed is relatively consistent, there's no panic because it's not yet windy, and it's not so light that you're going to lose steerage.
Lastly, a IMO a starboard boat cannot reasonably protest in a situation like this without being within a percentage of a boat length. Maybe it's 1/2 boatlength in big swell or swirling conditions, but it's probably 1/8th boatlength or less in champagne conditions IMO.

IMO, 10 seconds is plenty of time in most boats, and I might even argue 3-5 seconds is more reasonable in many boats. In bigger boats, the complexity grows, (as do the targets) but the boat must be crewed reasonably well, including being prepared for evasive maneuvers in a close situation.

As Calum notes, communication from both sides would be preferred here (even though it is not required). A hail of "Starboard" at 40-80 meters is useful. A hail of "I see you" at 30-70 meters (or in response) is also useful for assuaging the fears of the starboard boat.

As a jury we listen to the evidence of the parties and their explanation.

Firstly was there a change of course as if not there is no breach.

If there was was it a holywood or genuine, we get both.

Only experiance and your sailing can help you 

Port is always at rusk but starboerd often exagerates.

We the jury find the facts, and the better the jury the beter the decision gennerally.

A holywood is really a match racing term for a great show, but may not be genuine.

WS Case 50 is more on point. WS Case 123 is about when Starboard breaks RRS 14 while Port breaks RRS1 0, and Starboard's inability to be exonerated if there is damage.
Robert quotes Case 50's language referring to "a genuine and reasonable apprehension of collision" on the part of Starboard. Case 50 says that if S never changed course, P could not have broken RRS 10, as Michael says above. So that is the first question.
The heart of the problem are the questions, "What is reasonable?" and Reasonable to whom?"
I line up with John to say the jury is the judge of what is reasonable under the circumstances, which will involve questions about the type and maneuvering characters of the boats involved, relative speed and distance, and wind and sea conditions for a simple RRS 10 case. This translates into the time required for maneuvers and the space available to make them (see definition Room). These are the facts for which Michael is saying a good jury will look.
The point is to avoid making decisions (about Part 2 cases, anyway) based on semantic arguments whenever possible. That avoids over-empowering both nervous Nellies who bail out too early and parties who want to assert they are the final judge of what the rules require on the basis of their great experience or skill. 
Case facts say the closing range was 2 meters/second. If I were hearing this as 12 Metre case, I would not penalize P if S altered course at 80 meters. We are 40 seconds from contact at that point. However, I would keep it in mind how long it takes a 12 Meter to tack in judging how close is too close. A judge can benchmark that by watching the race and timing some tacks on the day, under existing conditions. We aren't too far from the point where P in a 12 M would be unable to complete its tack clear ahead without S needing to alter course. If it's an Etchells case, the protestor will find a very unsympathetic jury.

I think it is clear the case refers to reasonable to the starboard boat.
The jury are the arbitrer of facts, so they test what starboard says to see if they have established a reasonable apprehension. 
We cannot just accept the starboard boats account. We use our sailing experience, and as said the type of boat, and the distances. 
The case (50) helps the starboard boat,  but we have to be aware of, too early action to outweight abuse. 

Thank you all for your comments.  Yes, my question was meant to boil down to "what is reasonable?" and "reasonable to whom?"  Yes, I was quoting Case 50.

I wrote the facts to try to convey that S was too far out for P to know whether S had luffed to slow or, alternatively that the boats had changed relative speed for some other natural reason. 

As a low-level racer, really what I'm looking for is an understanding of any  rules of thumb or helpful conventionalities available to all boats. So your discussion of times and distances is helpful.

I'm still a little unclear on what, exactly, it means to hail "starboard".  Is it:  (a) Do you see me?  or is it (b) I feel the need to change course, so if you don't tack/duck immediately, you will have broken the rules.  I guess (b)?

Really really what I'm looking for is a way to decide whether/when my P-boat conscience can be clear after S-boat changes course when at a longish distance out.

Thanks.

Which will convince me better that you were anxious.

"I hailed Starboard. They ignored me. I hailed louder again shouting you need to tack too. They ignored me. I altered course"

"I wasn't sure if they saw me and I was anxious so I altered course"