Actually.... Mix it up.
Do a bunch of long ones as long as you can, till u feel tired, then finish off with as many short bursts till you are tired.
Or you can do one long one till you can hold it anymore and them right after do short bursts till you are tired.
I like the first one though. That's what I did.
Remember to do this every other day though. You need a rest day or two. Listen to your body.