Depends a lot on how much you play and how much of a tinkerer you are. I tend to play between 80 and 100 rounds a year between home and travel and I am definitely a tinkerer. That said, I never change anything without a proper fitting (and sometimes more than one depending on the results).

Driver/3-Wood - I will upgrade once I see a tangible improvement either in distance or accuracy. Generally every other release cycle is when I see enough cumulative gain ether through head or shaft or both. That said I did go from 917 to TS2 (same shaft) - it was that good!

Irons - same criteria but recently this has been each release cycle 716 AP2, 718 AP2 and now T100. Shaft change each time. Length change between 716 and 718 fittings.

Wedges - Will get fit each cycle for shaft, bounce and grind. Will change them every year though re-ordering the same versions between release cycles.

I don't have any hard and fast rules. If I find something that definitely will improve my game I try and figure out how it will fit in my budget. There have been times I have gone into the store ready to purchase the latest release only to find that I hit my old club just as well or better. I have also made the mistake of purchasing the latest version, and then gone back to the older version. Now, I am much more careful, and improvement has to be substantial before I will make a change.

I tend to upgrade my driver annually. When I find a 3 wood that works for me, it stays in the bag for a lonnnng time. Irons get replaced in my bag every 2-3 years. Wedges are a different story altogether. My most used wedge tends to be my most lofted, which current is my 58. It gets replaced every year. My 54 every other, and my gap every third season. This keeps the grooves able to perform to the max. As grooves wear, it effects flight as well as ability to stop the ball on the green. You may find that you lose distance as your grooves wear, as the ball can tend to ride the face and produce a higher flight, reducing distance control. Good luck finding the frequency that best effects both your game, and your budget.

Any thoughts on resharpening the grooves on your wedges rather than replacing ? I see there are tools for that, but am not sure about the quality of the job. I did have the grooves on an old set of irons regrooved by a clubmaker once back in early 2000 and it helped return sme spin to the ball.

I probably upgrade more than I should. Pretty well, when the new lines come out, I upgrade.
The only club I tend to keep longer in the bag than normal is my 3 wood. I only hit it maybe once every 4-5 rounds., but having said that I had a 917 F3 and bought a TS2 3 wood last month.

I tend to upgrade only if there is something tangibly better or if the clubs are really outdated. For the driver, I upgraded to the TS2 from another make as I was able to pick up serious distance and lower spin. For irons, I recently went from the 755's to 716 AP2's. Wedges went from Vokey 200's to SM7's.

Seems like I up grade every 2 to 3 years. I tend to buy a cycle behind. Being left handed that way I can buy demo's that haven't being hit pretty cheap. That said I did get a TS2 last year.

Just noticed that Justin Thomas still has some older clubs in his bag, 915 3 wood (after that shot at the 72nd hole on Maui it might get replaced soon) and a mix of SM 7 & 6 wedges