How often can a user rate an iOS app? There are 2 rating dialogs in the AppStore, one for the current version and one for all versions. Does that mean that a user can rate every update of an iOS app?
Yes. Current version ratings are reset after each update. Some apps even say in the update text "Please rate us each time we release an update".

If you previously rated an app, and a new version is released, if you go to rate it again your prior rating (and review text) will be pre-entered, but you can change it.

So, if you religiously review the app each time it gets updated, you could have "reviewed" each version, yet it's unlikely that your star votes add up and your new review seems to replace any/all older ones each time you update it.