my rule of thumb has been allow 7 minutes for every song, unless you are doing Master of Puppets or One or something, but, usually that works. and if you end up with an extra 10 minutes, NEVER try a free jam...extend the solo section to the last song about 4 choruses...it's better to be over a couple minutes than under a couple minutes.
probably between 10 and 15. that depends on how long your songs are. on average my bands songs are a little under 6 minutes so we usually don't have a problem.

but if you need to fill up time, do a really long ass solo, or a drum solo.
my band is here ------> myspace.com/weareattica