I think I like this idea that the rotation interval could be made proportional to length, for example doubling the interval with each additional character. Security standards already now acknowledge that forced yearly rotation is a net decrease in security, so this would incentivize users to pick the longest password for which they would tolerate the rotation interval. Is yearly rotation too annoying for you? For merely the effort of going from 12 -> 14 characters, you could make it 4 years instead, or 8 years, 16, and so on.