It really depends on the game. If you're up by a queen and a couple minor pieces, and on the verge of beating them anyway, time them out. It's bad practice, in my opinion, to time someone out when they are beating you. The game should be decided, to as great a degree as possible, on the chessboard.
I have to say that you just have to use your best judgement of that particular situation. If gameplay has been fairly quick, say about five moves a day, then after a week I feel it would be logical to time out. Likewise, I would grant much extra time if the vacation setting was on. Perhaps sending a reminder a few days before time out is applicable. Just bear in mind that time out is a win not earned, and I feel it should mainly be claimed when it seems the opponent has abandoned the game.