The site appreviewtimes.com croudsources reports from iOS developers to figure out the current average iOS/Mac apps review time:
Does apple reveal any information about the current review queue size?
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Apple doesn't say anything about the size of the queue, but they do give this information on review times on the developer website (emphasis mine):
Review times may vary by app. On average, 50% of apps are reviewed in 24 hours and over 90% are reviewed in 48 hours. If your submission is incomplete, review times may be further delayed or your app may be rejected.