Off hand, I think one of the last times it was 2-3 weeks or so, but there were instances of a week. Apple won't have a set rule that you cannot have some devices on a different build for a maximum number of time.
For example: iOS 15.1.1 was only for iPhone 12 & 13 on November 17th, and the next release to bring all devices in sync was 15.2 on December 13th.
iOS 14.2.1 was only for iPhone 12 mini, iPhone 12, iPhone 12 Pro, and iPhone 12 Pro Max on November 19th and then next common release was iOS 14.3 on December 13th. There's more, but that will do