The determining factors for me are travel time and my experience level with the particular client.  Motivation also plays a role:  if I am away from a home I will work longer hours to be more productive.  
Something no one has talked about is how to be more efficient.  Here's what I mean:  I am constantly looking for ways to shave time off my field time.  If I am stopped at a red light at the intersection in front of the station, I am already making observations while I wait.  During the 12 seconds it takes to pump my gas, I'm checking the paper towels, windshield fluids, and for bad lights.  While I wait in line for the cashier, I am looking at the register area, any required POP, and interior condition.  Before I leave my house, I have my route ordered sequentially, with my cheat sheet pre-filled with address, ID number and any notes about the location.  I make sure I have one credit card for every two shops; otherwise I already have a gift card in my wallet.  I have a fully charged backup battery and backup SD card.  The appropriate number of gas cans are in the trunk to accommodate the fuel purchases that won't fit in my tank.  If I forget something, I "write myself up" and punish myself by cutting something out of the budget for that month or depriving myself of something fun I had planned.  Acknowledging my own shortcomings is the only way I can correct them and become a better person!
I don't cut corners, but by being smart about my time, I've gotten my average time to the following:
Shell:  daytime 12 minutes or less; nighttime 15 minutes or less (shopping since 2009)
Exxon:  12 minutes or less (shopping for 10+ years)
BP:  15 minutes or less (shopping since 2009)
Valero:  8 minutes or less
When out of town, I've done 25 Shell audits in a day, broken up into two shopping periods so the reporting doesn't kill me.  If I'm doing local stores, I usually stop at 10.
Experience:  I've done hundreds, if not thousands, of each of these audits.  I don't need to review the guidelines.  I know every single scenario, every exception, and I take a few extra photos onsite that can be zoomed in just in case something dawns on me while I am doing my report.
Shopping since 1995; full-time since 2009.  Blogging about shopping on www.myfrugalmiser.com.