It depends on what has blocked you. In this case it was a spam bot that stopped me from posting comments into a group. Apparently there is a non-specific number of times you can post comments in a group in a short amount of time. The amount varies, but hovers around 150ish give or take 50 (at the time of my tests).
The ban appeared to be consistently set to about 19 hours at that time (May 2014). I've confirmed by continued testing in test groups and subsequent bans. However, Facebook developers are unable to give a solid set of numbers as they say it's controlled by a spam algorithm which changes based on server usage. So, 150 comments within about 3 minutes = ban for about 19 hours.