Should be in mileage, not time.
I'm one of those fools that change the seals before changing oil (which is the opportunity to change the fork oil).
I never only change the fork oil. And I might be wrong.
My little experience on standard forks says seals start to go bad after 50000-80000 km (changed the originals on 2 distinct new bikes).
So, For inverted fork, it can't be longer; my versys shows signs of leak (excessively wet forks) after 50000km (I do much offroad though).
I'm guessing seals go bad because of dirty oil.
Can you hint how much each job cost?
-Oil only is 1h labor+oil?
-Seals are another 1h + parts?
If you want to save the seals (hehehe) then you would have to change oils probably 2x faster, like 30000km.
So, 1h+oil every 30000km or 3h+oil+parts every 60000km.... I don't recall that seals are expensive at all. It's really the job that matters. I recall a 700$can bill for my old ninja 650.
For people doing it themselves, good for you!
For me, going to garage, I would really like to know how long seals could last if oil was changed more frequently.