I check monthly due to a constant dropping salinty. Between salt creep and skimmate I'm removing enogh salt that I can measure it after a month or two. (Don't do water changes unless I have a tank spawn.)
Also, what you check your salinity with can even more important than how often you check it. If you are using a swing arm hydrometer I would highly recommend checking it against someones refractometer or salinity meter. I had 2 hydrometers that I was taking an average of and turns out both were high.
Randy Holmes-Farley says that the salinity probes are as accurate if not a little more accurate than the refractometers. I've seen arguments both ways, but either are WAY better than a swing arm hydrometer. The key with both the probes and the refractometers though from what I understand is getting them properly calibrated and periodically checking the calibrations.
I check mine twice weekly. But..I bet it's cause I've only been at this less then a year. I'd bet that the people who have been at it longer check less frequently?? Just my thoughts
I went with monthly. I use a swing arm but also have a refractometer which I use to check the accuracy of the swing arm about once every 6 months.
Steve