I spend a lot of nights in the back yard and find that 60 to 90 minutes would be my minimum recommendation. A couple of hours would be better, depending on whether it's almost enough insulation or definitely not enough.

For instance, 20 degrees and windy with a fleece blanket? Not going to take me long to be certain that won't work. But if it's right on the cusp of enough or not it's going to be longer to make up my mind.