Yearly is normal for Apple, although I still don't understand the iPhone 8 yet.
Usually theres a 5, then 5S, 6, then 6S, etc. And apparently that's still true, there will be a 7S, but also an 8? I don't know either. I feel like Apple is losing their way a bit.
Their iPad lineup by model is getting all out of whack too. Their whole labeling/naming has completely lost me