I've had mine approx 3 years.
Jenny started in the size 0 plate and gradually increased up to a size 3. She then needed a new panel with a wider gullet to accommodate her increase in topline behind her withers. At this point she dropped to a size 2 (she needed more room behind withers but less in front if you know what I mean).
The fitter has been out >6x as she changed shape so dramatically initially and I'm so paranoid about Jen's back after all the trouble her previous saddle caused.
Now I tend to adjust the saddle mostly myself under the guidance of my fitter - Jenny fluctuates a little with the headplate size through winter to summer and is also about due a minor air adjustment as she's a bit more symmetrical than she was 6 mths ago.
I've had my wow 2 years. I have my fitter out every 6 weeks and invariably I need the air tweaked or the head plate changed (my boy fluctuates between a 2 and a 3 depending on the time of the year). It is definately worth having it checked regularly as in my boy's case his musclature has really changed since as he has progresed up the levels in dressage. I change my flaps at least once a week so we can go jumping.
I bought mine last year before Wills was diagnosed with kissing spine, it'll be interesting to see hows he changed shape when I finally get the company out when he's back in work.
It's the only saddle that does not give me back ache.
I've had my WOW just over a year and I love it, it's not the saddle in particular but I love the way you can change it as your horse alters shape.
I have the saddle fitter every 2 - 3 months or when I feel it needs it, we regularly change the headplate (my horse changes between a 4 & 5) and occasionally adjust the air.
I regularly change the headplate myself and also the flaps. I've had to buy another pair of panels for my boy as he changed shape dramatically but this took about 10 months and he's still changing.
I've just bought another one now for my girl as she had outgrown her saddle and I wouldn't have any other saddle.