I'm guessing the ICE stays on during this situation, so you don't run the the hybrid battery down. My mpg at a snails pace was 25 mpg.
Once warmed up, the A/C can run without the engine until the Big Battery gets low, (15 minutes?) then the engine will come on to recharge the battery. During warmup, the Prius has different priorities and will keep the engine running to warm it up. http://drivesmartwi.com/uploads/DSW_Toyota_Prius_Initial_Stages_of_Warm-Up_Wayne_Revised_2_.pdf
Thanks, makes since. I turned it off last part of slow trip thru downtown . It was overcast and cooler so the mpg went back up.