Dawn, I've done hundreds of these. I always allow about 40 minutes for the audit (much less, about 15 minutes, for the mystery shop alone). You can do as many a day as you can get to (I have personally done 7 or 8 in one day). For Marketforce, I like that they reimburse the relatively tiny gas purchase, and a small amount for the required "inside" purchase, and the guidelines are straight-forward and the reports are basically simple.
I would recommend printing out the guidelines in full for the first one, just so you don't miss some critical item. You might want to start with just the mystery shop ones, and do the audits later, after you're more familiar with them.