Don't worry, the trichomes can't dry out lol
As Zandor said- a gentle breeze, just enough to keep the air circulating around the plants. Don't have the fan blasting directly on them though, aim the fan at a wall or something. The fans are best left on 24/7 (though this isn't always practical for many people because of noise issues). Look at it this way - the breeze outdoors never stops does it? If noise isn't a problem for you then keep em on 24/7.