I usually try to set aside at least half an hour in any given day for each game I'm playing in via PBP. Often, given the particular activities of the game at the time, and the posting rate of other players, I'll end up spending substantially less time than that, having little to do in the game at that time. Other times, I might spend as much as an hour or two, if I'm composing more extensive and detailed posts, receiving prompt replies, and then replying again in turn.
More generally, though, topics such as these should be discussed with the (prospective) gaming group beforehand to arrive at mutually agreeable expectations for behaviour.
