I have several gents who see me on a "rotation" of sorts, similar to what you describe. Most of them don't write reviews at all. The few who do, probably will write a new review once every 4-6 months.
In my personal opinion, as long as I get a "fresh" review about once a month, I don't particularly care whether or not I get them anymore. Once you have established yourself, they just aren't AS critical as they once were.