How long are classifications valid?

From the 2019-2020 version of the USA Fencing Athlete Handbook:

2.1.4 How long is a classification valid?
Classifications are valid for four seasons after they are earned. If, after four seasons, the classification
has not been re-earned, it then reverts to the next lowest letter and the then current year. Such
change will be effective at the beginning of the new fencing season – August 1. For example, if you
earned a B in 2010 (B10) and did not re-earn your B for four more years, your B became a C14 on
August 1, 2014. If you then did not re-earn your C14 for four more years, it becomes a D18 on August
1, 2019.

Check out Section 2.1 of the USA Fencing Athlete Handbook more information about classifications.