How long should I use a fan on indoor seedlings per day?

I have some trays set up with seedlings about 3 inches tall, they aren't leggy and I use grow lights, but I did experience some damping off that killed some dill, a few squash plants, and some snapdragons. I heard a fan can help with circulation and building stem strength. How long should I leave my fan on? Is it better during the day or at night when the lights are off and the plants are resting? Should I leave the fan on an hour? Six hours? 12 hours? Thanks!