The answer is that you don't have to test it very often under the circumstances you describe. If you don't have a lot of splash-out and you use a cartridge filter which never requires backwashing (cleaning, yes, but washing using pool water, no), then you are correct that the CYA level will stay pretty constant throughout a swimming season and I have found that to be the case in my own pool.

However, over a winter, you may find that your CYA levels drop. If you keep your pool covered during the winter, then some forms of anaerobic bacteria will breakdown CYA. Of course, that implies that your chlorine level may drop at some point during the winter -- if you keep your chlorine level up then you will likely preserve your CYA as well, but some people just let their pool water sort of go "bad" over the winter and then deal with cleaning it up at the start of pool season.

Generally speaking, you should test your chlorine levels (FC, TC to get CC) most frequently (say, daily, unless you have a pool cover), then your pH level (once or twice a week is usually plenty unless you use Tri-Chlor -- bad for other reasons -- or your pH rises quickly and you add acid frequently), then your TA (monthly is usually OK unless you have a lot of pH rise and add acid frequently), and finally your CYA (twice a season may do it unless you have a sand or DE filter with backwashing and/or lots of splash-out).

[EDIT] JohnT wrote his post as I was writing mine so sorry for the duplicate information [END-EDIT]

Richard