Yeah I don't think we've ever seen only one generation of a new model. Minimum I think is two. Especially considering Apple considers the Air a substantially new model and not just a slight variation like the Mini or Plus (hence not 17 Air), so Apple probably invested more R and D in the Air and wouldn't want to kill it off so quickly.
The only way MAYBE is if the Air was planned as just a one generation stepping stone to the folding iPhone. But that doesn't seem likely to me.
And usually the first generation of a truly new model won't get a number, whether it's updated yearly or not. Both the original iPhone and the iPhone SE didn't have a number. So I don't think we can predict update frequency right now based on that.
Edit-
There is one exception example of a one generation model that came to mind, but it wasn't an iPhone--it was the iMac Pro. And I believe that was just a stop gap due to Apple needing time to revamp the Mac Pro.