At the start of play, about a minute on a harder to plan kingdom, but typically I'm away in 10 seconds or so (but sometimes to my own detriment, I should probably spend 30 seconds before starting so I don't do stupid things like forget that pawn can give +buy).
If you're going to do some heavy thinking though, it's courtesy to say at least 'hi' so the opponent knows you're there and paying attention. If you haven't said anything or done anything after 30 seconds, I'll probably prompt.
After play begins, I don't like any breaks longer than 20 seconds or so without a 'brb' or 'thinking', or an apology on return if something unforeseen happens (connection problem, cat jumped on computer, etc) - I always try to explain/apologise, but I'm not normally particularly bothered if somebody else takes an unexplained 60 second break in the middle of the game.
Also note that people on crappier internet connections may be posting '?' or '.' some other character just to make sure that the server echos it back and it's not their session that's the problem if you're wondering about what appears to be rude prompting in others.