This was something I always wondered myself actually, since TTT was never really the gamemode I played ofter. I figured it was when there were maybe 2 or 3 people left in the round, and they hold it up for 4 or 5 minutes, but I guess it's always good to make sure.
Not really, still depends.
http://wiki.randomgs.com/index.php?title=Server_RulesThere's still no official rule against delaying the round in TTT. But the whole point is that if something's occurring in the servers which is causing a majority of players to be unable to play the game, unable to enjoy it, upsetting players possibly making them leave, etc, then something should most likely be done to solve the situation. So if players delaying becomes a problem in any way previously mentioned and players are noticing it, might be a good idea to warn them earlier and possibly kick a minute or so after if no effort is being made.
Really though delaying is also simply a part of the game somewhat inteded. In some cases delaying can be necessary but to an extent so I wouldn't really just go by a specified guideline of "warn at 2 or 3 minutes kick at 3 or 4" because in this case every situation has its own circumstances.
For example, if someone was delaying the round for maybe even simply 1 or 2 minutes but everyone's dead and the last innocent is simply sitting in a hollow prop completely invisible and the traitor hasn't bought a radar and has been wandering mindlessly, then I would warn pretty quickly and kick(or explode in my case) even quicker if they make no response to a warning.