Walter once bubbled...
I buy when I need something. I'm not on a schedule. I have pretty much everything I need. I buy when I need to replace something worn out, broken, used up or lost.
That is also the way it works for me.
At the beginning of winter, each year, I have my regulators serviced, and I clean and inspect all my own other gear. That is when I will do most of the major replacing. The two week interval that the regulators spend at the scuba shop are a perfect time to repair or replace all the other gear as well.
Replace the batteries in the computers. Replace the straps on the instruments and gear. Check and replace valves as needed. Check and replace seals as needed.
Recently bought a new compass, because mine was too scratched up and worn to work very well. The new compass should last 5 to 10 years, like the old one did. That was the only new thing I bought for myself in the past year.
A year ago I bought a safety sausage for a dive trip to Fiji. Locally I do not have much use for a safety sausage, since most of my dives are beach dives or non-current boat dives. But with Fiji's fast currents, a sausage was a good idea, if not indespensible.
That amounts to once each year for me.