You can get inexpensive digital timers that will let you get down to 1 minute increments, but the problem is most of them have a limited number of settings. One I have has 24 settings. But remember that turning the fan on, and then turning it off are each seperate events, so you end up with only 12 on/off cycles per day. Might work if you do it for 10 minutes per hour or something.
This one is similar to what I have, but it only does 20 events per day (10 on/off cycles).
You can find the ones that would allow you to set things like 2 minutes every 30 (i.e. 2 on 28 off), but the cost of those types of cycle timers starts getting a lot higher. $50 - $100+