5-10 ideally, it allows you to change practically everything within the story that isn't long term social shift or on geological timescales.
Longer means you lose more and more connection to all the current cast the longer you make the skip.
Everything longer than 50 years might as well be a complete reset of the franchise, outside of a few elf characters, which you absolutely don't want in a franchise that is 90% built on one game of a series of 3.