In this video I change from low to high before a long, gradual descent with a few steeper drops. Even with the rear fender in the way and the bike's weird dropouts that force me to play around with the quick release adjustment, the change takes about a minute.
On a bike with no fenders and decent forged dropouts I could make the switch in 30 seconds on a good day.
By the way, it didn't really rain that day even though the forecast sounded certain enough to get me to go with the rain bike. That's okay. Occasional fixed-gear rides really help smooth out your pedaling technique and expand your power range in any given gear.