Like I said, get the resources I pointed out, it will give you a lot of information.
There is no set time, because there are always situations that will come up that will screw up your plans. For instance, you just begin your cups and balls routine, and the food comes out. You can't go on with a 7 minute routine. You need to finish up quick.
Each effect has to be self-contained. If it isn't, you will be in trouble. Because when the food comes out, you have to leave. You can't make the wait person stand there while you finish up your routine. The food would get could, your audience would get annoyed, the wait person would get annoyed, and you will probably be fired.
Also, sometimes you will do a longer routine. If it's a slow night, you can do longer routines. If it's a fast night, you want to do short routines. If there is a problem in the kitchen, and someone's order will take longer then normal, you can do a longer routine, to keep the people's mind off their food.
So really, the best idea is to get the sources that I suggested. There are also other sources, but those will give you a good introduction into resturaunt magic.