Tony has the right answer..

You have to try and keep a pattern or club owners will take advantage of you..40/20...45/15....and 30/15 are standard..But I agree with Tony , don't even watch the clock when people are responding to your music..A packed dance floor will rekindle your energy better than any break...

If you don't believe me ..think about the times you play to an empty or dead crowd..How long were those gigs even with breaks..

I don't think I had an audience smaller than 150 people in a few years,, Matter a fact lately our crowds are 250-300 people...We have a great time on stage,,,playing an hour or more a set...

A couple weeks ago we played an outdoor shore gig to more than 2,000 people..The gig was only an hour and a half..but we could have played straight thru for 4 hours at the excitement level..

But to answer your question 40/20 is standard and most places prefer that you take care of the break music..Clubs don't make enough money to be bothered with jukeboxes..