Yep, you're wrong. 30S to late 60s, battledress. Late 60s to early 70s Green Combats, which were then made DPM and went on until the early 80s. Between then and combat 95 we had about 3 different types of combat kit, so it's not exactly every 10 years is it? Not to mention the boots changing every five minutes after having the DMS boot for the whole of the cold war (almost!).
If you are thinking of webbing? 38, 44, 58, 90(ish) for PLCE and it hasn't changed exept for the colour since.
So needless to say there is no set time limit for kit to be in service. It gets changed when somebody finds a replacement that does the job and isn't too expensive.