Timing of fan in small grow area

[ladyslipper]

Hi guys, My grow stand has a light plastic covering on the front and sides and a much heavier plastic on the back so I can reach in a spray water on my phals without worry of spraying water all over my livingroom. I know there is a concern about having a humid environment without good air circulation so I am going to try a USB fan (looks like this Elite Mini USB Lite-N-Fan Set at Memory Express) as from what I understand the breeze is not very strong.

My question is do you keep fans going all the time; or for a few hours? I don't know how loud this fan will be so that will have an impact on how long I actually end up running the fan.

Thanks for any info guys!

mary