Weekly
I usually check mine once a week. I have them on trails by my stand sites so I also take the opportunity to spray a little DEET on a couple trees near the stands when I'm out checking the cams in late summer. That first week or two of the season I'll have plenty of bug dope on, so my theory is to get them used to catching a whiff of it. It might not help, but at least it hasn't seemed to hurt.