The time editors spend is highly varied, but I think many of the editors who are active spend 5-20 hours a week, some of that editing and some reading and communicating in the forums. A lot read forums during lunch, and then edit an hour or two at night, or for a chunk of time on the weekends.
Warning - it is addictive! You'll probably find more time than you thought you had because, darn it, you are just going to find 5 more sites to list in that category, or catch up on your unreviewed, or adopt a sorely neglected category, or build a brand-new one from scratch, or figure out what happened to that great website you once loved, or ...