I do all of my trainer work in the garage and the temps are anywhere from 35 to 60 degrees. No fan so far and I've been able to go as long as 3 hours. 2-3 headbands and a hand towel keep the majority of sweat from the floor and my bike.

I took the rig with me on a business trip and set up in my hotel room a couple of weeks ago. The first workout was 2 hours and I thought I would die. Heartrate was nearly 10bmp higher for a given effort level! Needless to say, I bought a fan the next day and was amazed at the difference it makes.