That really depends on what you want to get from it. For me, I just like to go pick up my race numbers/chips, get my goody bag, and check out the race merchandise. I don't spend much time looking at all the vendors. I have been to a bunch of race expos and I have seen all the same vendors over and over so I just skip them.
Last year, we were in and out of the expo in less than an hour. (THat was first thing friday morning--there were no lines for packet pickup) I am sure if you looked around at everything you could spend a couple of hours there easily.
You can go back as many times as you want to check out the vendors/merchandise etc.