We've only been doing it maybe once or twice a year so far, but I was thinking a little more frequently would be good. However, I don't want to be too frequent.

You really shouldn't be editing them. If they are static pages and they are performing, then I wouldn't touch a thing. The only time you start "tweaking" <title> elements is when you are fine tuning the targeting of a very specific keyword phrase and a very specific position. Outside of that, once you set up a static page, it requires very little maintenance.

Dynamic pages are another animal. Depending on their nature, they could either be static or dynamic or a combination of both.

But, when it comes to page titles, those usually remain static. You really don't want to fiddle with those unless you absolutely have to.