I'm not sure whats recomended - but this is what I use.

During early stages of growth I have an oscilating fan (sorry for spelling - just lit up) on for 18 hours on the same timer as the lights. During the dark period it goes off.

Every 3rd day I keep the fan on 24 hours, until eventually during flower I have it running 24/7.

This may not be right as I am fairly new to growing - but hope this helps slightly.