I've never even heard of a 91 day rule / guideline before. Perhaps it has more to do with AORs than one-at-a-time applying?

However, there are some bank-specific rules. The ones I know are the Citi ones: At least 8+ days between your first app and your second one, and at least 65+ days between your first app and your third one (ie, no more than 2 Citi apps in any 65-day period).

Then there are inquiry rules that are also bank-specific. The one I know is Citi's, where they typically will reject you if you have more than 6 hard pulls showing in the past 6 months on whichever bureau's report they happen to pull.

Please note that 91+91 days is essentially 6 months, so part of it may be to space AOR pulls far enough part to not upset banks who care only about the last 6 months of pulls. But obviously this would not matter if you're not AORing and only applying for a few cards every 6 months anyway.

AOR = App-o-Rama = applying for a bunch of cards at one time. Some people do an AOR, then take a rest, then do another AOR, then take a rest, etc. For them, because they're applying for some many cards at one time, the timing between AORs can be critical. For those who only apply for one card at a time and only a handful of cards in any 6 month period, most of these "rules" that AORer's come up with are irrelevant.