I have the a Whipserflo also. Mine is 3 years old and I just replaced the seal a few weeks ago for the second time. I don't know what the normal life expectancy of a seal is but this is my filter pump, used 3 hours a day in the winter and 10 - 12 hours a day in the summer. I'm in Northern CA and I don't close either so this pump is used everyday. So for me it seems 1 - 1.5 years is what I get out of it. I don't know if that is normal or not, but seems like a lot of hours.
The first time I changed the seal, there was also leaking from the housing also. Housing bolts were loose and the factory had mis-installed the rubber housing seal, it was not seated completely in the groove and had been pinched a little. I made sure the new one was seated correctly. However both times I could tell the seal had been leaking because the metal parts of the seal had rust and water stains on it.
Mine never loses prime after a clean out either. Takes maybe 10 - 15 seconds to completely fill the basket but I don't believe that is dry as water is coming in a little at first then more until the basket area is filled.
The seal is made out of a ceramic material (not rubber), which seems tough to me and I think only a little water is needed to keep it cool and undamaged. Early last year a friend's pump lost prime (due to air leaks) and the small amount of water left in the basket area got so hot the basket got soft and warped, but his seal doesn't leak.
In the seal there is a spring that pushes the ceramic parts together, in the old ones I took off I noticed the spring seemed weak, it may have been my imagination but I think it was easier to compress a little on one side than the other, but I don't know if for sure. So maybe the spring gets weak and causes the leakage??
PS It is not that big deal to change it yourself if you have a few hand tools and a bit of time. The seal itself is around $12, but then you need to source the other o-rings, seals and lube. I would change all the rubber o-rings and gaskets at this time too, I noticed on mine all the rubber was deteriorating - all left a black oily marks on my fingers. Why not do the complete job while you have it out of service? I found a kit that includes everything needed:
Pac Fab Whisperflo O-Ring Kit Go-Kit 32 for $30US +shipping.
Do it before the water gets into the motor bearings and causes some real expensive damage.