If it's an opening set, find out what is wanted (usually no more than 30 minutes), and stick to it. Consider that limitation an absolute, especially if you want to be asked back.

Coffee house venue as main attraction, I agree with the 45 - 50 minute set, 20 minute break idea; but be flexible. Audience members may keep you chatting (& buying CDs) longer. This is generally a good thing.

Best "we'll be back soon" line I've heard (I'm using it myself, & I wish I knew the originator, because I do like to give credit): "We're gonna take a 5 minute break for 10 minutes & we'll see you in half an hour!"