The seal will last a very long time, as long as you don't remove the dipstick.
Unfortunately, I have to remove the dipstick on my car, in order to get access to the timing belt.
So every 4-5 years, I need a new seal, but as I just recently have changed the belt, I will not need the seal the next 3-4 years...!!
Anyway, I'm aware of the fact that dealers can locate single parts, but this part is nowhere to be found in Holland...!
Even another Mod on this board has tried to locate one seal for me in the UK, but no luck as well...!