nottatrain has it spot on IME, although on the few occasions I have detrained from the TGS on an HST to the cess trackside it's been nearly 2 metres where the cess slopes away!
It would be madness to detrain more than a handful of passengers for more than a few hundred metres walk IME/IMHO, you are simply asking for trouble with slips/trips/falls even IF all passengers had no mobility issues and were likely to accept instruction from the TM/staff.
I had 2 people detrain through a window once when we were in sight of a platform (think it was Earley IIRC?) when delayed for around 45 minutes once, frankly they were insane- it must've been nearly 2.5 metres head 1st, and if the signal had 'come off' whilst they were doing it they had a real chance of falling under the train.
Even then, Earley was unstaffed and unlikely to have taxis waiting, so since we moved less than 20 minutes later (towards Reading) they almost certainly delayed themselves even more.
I think the arguments for 'how long is too long' could go on forever: if there were to be 'guideline times' (that would eventually inevitably become public knowledge btw) they would have to be 'per route/per time of day or night/per time of year/per weather conditions/per loading etc etc etc: how long is that piece of string?
Personally I think the FGW 6 hours business was too long as an 'off the cuff' opinion: however I'd have to know what resources FGW had, the weather, the time, the loading, the experience of the staff, the provision of managers/trained staff and the exact location to even start to criticise properly: that isn't going to happen is it?