I change out only if I find a handgun that's more reliable/accurate/concealable/better stopping power for me. In the past two years that I've been carrying, I've gone through a Walther PPK/S (broke the day after I got my CCW, so I carried my brother's bulky G23 briefly), a Kahr K40, a .357 Taurus 617 (more accurate with it than my Kahr), and for the past month or so, a .45auto Para Ordnance C6.45 LDA.
Keeping with the same gun on a regular basis keeps me familiar with it, even though I AM familiar with all my guns. In the heat of the moment, for instance, I might "forget to disengage the manual safety" on my Para if I keep swapping around with my K40 on a regular basis.
I do, however,practice with it in the mirror (unloaded, of course!), drawing it and flicking the safety off, etc. when I'm not shooting at the range.