There has been lots of controversy over breaks, the length of a break, and whether or not music should be played.
Much of this depends upon the individual venue. Some feel a rule of thumb is 40 minutes of playing time, 20 minutes of break time, especially when the job is four hours long.
As for the jukebox, I've always tried to keep them out of the picture if possible--particularly if you're playing in a bar or niteclub. By allowing someone to fire up the jukebox during breaks you are allowing some drunken fool to put several dollars in the machine, which always translates into a lengthy run of music he or she likes to play when plastered. This ticks off the person that hired you because you can't stop the jukebox and go back to performing.
The solution is simple. Fire up a series of MP3s or midi files to cover the music while you take a short break. In doing this you are in total control of the music. If some drunken idiot still persists in putting his or her money in the jukebox, which sometimes happens, talk with the ower or manager about unplugging the jukebox while you are playing. They are usually very cooperative about this.
Good Luck,
Gary
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Travlin' Easy