I don't have a formula and there are a number of factors such as traffic volume and type plus the level of instrumentation that would affect this.
Assuming you are referring to session storage (i.e. PurePaths) what I would try in your case is (if it is an existing profile and you are looking to extend storage) right click -> details to see how far back you are storing data with your current level of storage. Then do some arithmetic to determine how much storage would be required to have an entire year's worth of data.
Note that personally I wouldn't find too much value in having a year's worth of data to go back to. I've never been in a situation where that was necessary and long term charting in the performance warehouse is more useful in long term situations, generally a month's worth of PurePaths is more than adequate.
James
