A SEP Class Rating is valid for 24 months.
To revalidate:
Fly 12 hours in the 12 months preceding the expiry date, including 6 hours PIC, 12 take-offs and landings, and 1 hour with an instructor.
OR
Do a Proficiency Check with an examiner within 3 months of expiry.
Don't forget to get your licence signed before the expiry date, or it doesn't matter what you did - none of it will count!
If the rating lapses, you renew it by doing a Skill Test with an examiner. This has the same content as the Proficiency Check; it's only the name which has changed!
I just got back from an FI revalidation seminar - might as well pass on some of the stuff we went over!