On our scheme, when you get to a certain number of years such as say 9, that is the maximum discount so even if you had 29 years, it would still be the same premium.
However, what I would suggest, & this is what I do, is keep hold of each renewal notice so that you have proof of your history. I do agree it's nice to have a record but once you are up to the maximum, anything extra doesn't count for much!