Its been at least two weeks since my last one. I don't keep too many apps on my phone, but use lots of music, pics, videos, email, yahoo and google messenger. Try to keep at least 12 Meg free. I totally love this phone.
Battery pulls are required more if you have a lot of apps or you install and uninstall apps a lot. This of course affects your free memory and the battery pull clears out memory. Also if you upgrade to a newer version of your device os then you will (in most cases) get better performance and memory management out of your device which translates into fewer pulls. Its been 1 month now I believe since my last pull, and the reason I pulled was because I had uninstall an app.
I don't do battery pulls. I usually reset it when I uninstall or install something. Other than that, I can't remember the last time I have done a battery pull when I wasn't prompted. Yay for the 8900 =)
I don't battery pull anymore since I dumped default blackberry web browser and switched to Opera Mini... Now I just use the blackberry app "Soft Reset" when the memory leaks drop my memory to 20 meg and then I'm back to 28 meg...