Tiziano, great question.  I was actually on the PC for the protest that resulted in the published US Appeal 124

We wrestled with that question as a PC in the hearing and during the appeal process. 

We looked at the available Cases/Appeals specifically as cited in the Appeal, US Sailing appeals US46, US67 and US82.  The result of that appeals process produced US 124.  When taken together, these 4 US Appeals (including US124)  start to paint a picture.   

Now, with these in mind I ask the questions;

Then you ask is all that the first reasonable opportunity?

In the UK, RYA Case RYA1999-01 that says the flag must be "close at hand" to fly, but the US doesn't have that.  

I'd recommend reading the 4 US Appeals and see if they help you bound the problem in your mind. 

Ask “how long does it take to pick up a winch handle”?   A Protest Flag is a similar piece of equipment which should be kept close at hand . 

Our protest committee requires immediate notification. I always hail "protest" immediately after the issue - I can always reconsider afterwards. When I raced a bigger boat, we kept the red flag rolled up on the shroud so it was available immediately. 

Mike, RYA 1999-01 is very interesting as it highlights the diff approaches in the US and UK. 

First the onus on the flag being "at hand".  We don't have that really based on the US Appeals. 

Then, another divergence is in the hearing, where it states that exploration the promptness of the flag is not necessary if the protestee makes no objection. 

That would imply a process in the hearing like:

PC to Protestor: Did you fly a flag?
Protestor: Yes
PC to Protestee: Do you have any objection regarding the flying of the flag or its timeliness?
Protestee: No

... end of flag inquiry. 

That's not common process in the US that I've experienced. 

I've had situations in high school dinghy races where the incident caused a capsize. What is the first reasonable opportunity then? As the boat is going over? When the boat is in the water? After you've ascertained that your crew is okay? Once you have righted the boat? Flying a flag is not an issue here, but safety certainly is, and the likelihood of the other boat still being in the vicinity to hear a hail is not high. Although I know that actually hearing the hail is not required, if the other boat is not in the area, what's the point?

Protest flag timing again?
My approach over the last 32 years as an IJ (and before that as a National Judge) has been:  Safety of the Crew; Safety of the Boat; Flag!  In most cases, check one and two are done in a glance.  I used to say (way back when even dinghies had to fly a protest flag), if single handed boats could do it within seconds, surely a multi-person boat could mange the same thing. 
The comment above, "How long does it take to find a winch handle?" is a good example.

In sailing races, the term "Reasonable Opportunity” refers to the expectation that a competitor must take action—such as protesting or displaying a required signal—at the first practical moment after an incident occurs. This concept is particularly relevant, where a boat must hail "Protest" and, if required, display a red flag “as soon as reasonably possible”

Factors that influence what is considered a "reasonable opportunity" include:

Weather conditions** (e.g., strong winds or rough seas may delay actions).

Crew availability (whether sailors are occupied with critical boat operations).

Location of required equipment (such as the red protest flag).

Tactical situations (e.g., approaching a finish line or navigating a congested mark rounding).

Protest panels assess whether a boat complied with this requirement based on these circumstances. If a boat fails to act within a reasonable timeframe, its protest may be deemed invalid 

I don't think you can have a hard and fast time for "reasonable", and that's deliberate. It's a term of art from the common law, and it's always contextual about what is reasonably to be expected from a reasonable person in all the circumstances. Using a term like that in the RRS isn't accidental in my view, and is intended to give room to assess the individual circumstances.

The RYA position only follows good sense and general sailing practice.
A flag chan be furled on the backstay or shroud, and secured with tape. This by just leaning over it can be unfurled in seconds.
The teas then is relative to this. So for instance if you have to go belw toa locket to get the fag and this takes 30 seconds, relative to the above it is slower so not at the first reasonable opportunity.

The rest is relative to a prompt response not quick because of the relative location of the flag on a particular boat.

Yes on Validity, if all is in order on the protest form little need be asked unless it is raised by the other boat. Asking if there is any complaint identifies to issue so you cam move on.

On appeal generally if a boat has not challenged the validity of the flag or hail initially at the hearing, then there is no act or omission or is partly that boats fault for not raising it. No redress, and the reasons just follow the rules.

If not it starts to appear in all complaints, Reopenings and appeals.