We use Tiffany and they recently changed from 30 to 20 minutes but I haven't had any problem. I think they just want you to keep it reasonable. Even without knowing the store your only buying a few things to get through the trip so it shouldn't take long. I make a list and group the items the way they would be found in my supermarket here. Its generally close enough to make things quik for me. If you have more than one adult you can always split up for max efficiency.
Personally I always tip a bit more with the stop as the driver could have made another run, YMMV.
TJ