I agree with Gary's 40/20 rule with a band. For solo work I typically do 45/15. Depending on the night, I might do 60 for the first set and shorten the last set.

As for music during breaks, I think it's OK but you should try to control what it is. used to play in an 8 piece band that did big band stuff. During the breaks the band leader would play a CD with Count Basie, Ellington, etc. I thought it was a mistake to have the CD sound better/fuller then the live band. I think a small/light combo CD would have been more appropriate. Choose the background music carefully. You should control the entertainment from your first note until you start to pack up.