I agree with the timeframe alosso has pointed out regarding from level 90 to 97.....but I believe the clubs available at level 90 is really all you need. From there you may be buying a slightly slower meter.
I still use my R11's sometimes and the difference to the 97 Nike Irons, apart from meter speed, is of negligible benefit.
Interestingly, in another thread Icon said the game has not become harder in the last 5 years, except that the courses have become more difficult. I agree with this but what has improved is the attributes of the clubs, hence why we see such low scoring.
Yes WGT will keep releasing new and slightly better clubs as this is the business/upgrade model, but I do not see real game changers being released. A top player will upgrade, not out of need, but because they can and it might give a slight edge,
As an average player who will never contend for anything than a wooden spoon, I see little point in obligatory upgrades that will not improve my scoring.