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.