Block Heater--how long before warm?

[JBG]

Did something crazy and ran 150+ feet of extension cord from my 2nd floor apt down to my wagon, which is now covered and surrounded in about 1 foot of "snow" (mostly ice and water, some snow underneath....horrible stuff).

Anyway, I'm trying to get the block warm enough to see if it'll start. It got above freezing today, barely, and shouldn't be too much colder in a few hours. My question is, how long should I wait before trying to turn over the engine? How long does it take the block to warm up? I seem to remember reading something like 3 hours, maybe less.

Thanks,