Maybe 5 times a year to test the strength and range of my transmission out of about 40 "sorties" per year from the slip.
Less than two years ago, after going up the mast myself, I noticed that the housing for the connector to my VHF antenna coming out of the mast had cracked from weather and had become accessible to water. I had it replaced; naturally, I wanted to check the transmission after that, etc. So, there can be many reasons. Obviously, if you chat frequently on your VHF to your sailing buddies you do not need to check it often by request. Otherwise, it's good practice to make periodic checks, in my opinion. There being about 10,000 boaters with VHF radios in southern California, even at only 5% doing it once per year, there would be between one and two checks a day on Ch 16 (or where ever). It would be interesting to get Sea Tow's data on the check rate for the LA area.