1st Refresh - 273 days.
2nd Refresh - 237 days.
3rd Refresh - 499 days.
We are currently at 272 days since that last refresh. The 3rd refresh's length is a question mark. It could be bad data, or it could be a sign of lengthening cycles (although not terribly likely that it means we should be seeing 18 month cycles regularly).
Reality seems to say something different than you claim it does. Did you even bother to reference actual numbers before you made your "always at least a year" claim?
It would make sense for them to release on Sandy Bridge. They can't get away with letting other ultraportable PC's come out while they keep a Core 2 Duo. They'll look like a real joke selling Core 2 Duos for much longer in a notebook which is supposed to be a premium product (basically all of Apple's products are supposed to be premium in their marketplace).
I could see them skipping Ivy Bridge, though, if the integrated graphics isn't any good. They could stick with Sandy Bridge for a while until they find a useful and popular choice for better graphics on the Macbook Air. Quite honestly the processing power will be great on the new Air, so the next refresh after that will have to work on graphics by a fair amount... otherwise more processing power will be rather pointless to be honest (obviously it's never pointless exactly... but to the average consumer, and even to most power users, graphics power needs to complement the processing power).