If at all possible, one should plan a field trial with a day or two of flexible time. This is because things will typically take longer than planned, and because we all know you can't predict the weather. One should be prepared for both very cold and very hot weather.
One should consider beforehand just how bad the weather needs to get before it is not feasible to continue with a trial. Experience with the local weather conditions in the local area can be helpful here. If the wind starts to blow or it begins to rain, how many days in a row is it likely to blow or rain?