I have submitted 3 apps. One app took 10 days before it was rejected for reasons I don't necessarily agree with, but whatever, I re-wrote the majority of the app and resubmitted, but it's been just about another 10 days
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Second App took about 4 business days (tack on a weekend in there and it was 6) for them to notice a bug that I immediately fixed and re-submitted within hours, tick tock another 3 days and a weekend added on and still in review... I'm hoping that it will be approved today or tomorrow...
Third app got rejected in 3 days because my icons didn't match - bad instructions on their part in my opinion... anyway, I fixed the icons, and it was live 2 days later.
I think it has to do with complexity. The app that got released the quickest was, in some respects, the most basic, just using Nav controllers and tables etc... the one with the 10 days-to-reject used a lot more CG stuff and different api's etc. The one that took about 4 days for them to notice the bug, again is a pretty simple one, animating a bunch of static UI views for the most part...
Like I said, I think it depends on the complexity, if there is anything that could be "risky" about the app, and if you are using multiple api's or whatever...