My answer varies both with the time of year and with the location of particular cams, and more recently, whether I can determine the cam seems to be working properly without my visiting it or without touching it.
I see no need to check cams over salt licks any more often than monthly if the cams are working properly. It's just unfortunate that you might need to check more often just to be sure they're working. During deer season, I like to check cams going or coming from a hunting location --- so those cams get checked whenever my hunting route takes me near them. I tend to do this checking more when walking out than walking in.
If at all possible, check your cams near mid-day, or whenever you believe is the time least likely for deer to be nearby, or a time of day you would never or seldom hunt near that location. For example, a cam located near a food plot (good evening stand location) might be checked around mid-morning and there be no deer knowing you're there. Late evening would be the worst time to check it. One reason I generally prefer around mid-day is because sound, scent, and sight are typically diminished or dissipated more around mid-day, and this also typically allows more time to elapse (more of your scent to dissipate) before any deer come near there.