The validity of the SEP class rating is two years from the moment the examiner signs the paperwork (rounded up to the nearest month I believe).
From the date of test which might not be the date of signing. It is valid until midnight of the day before the second anniversary of the test. i.e. test date - 1 day. On re-validation it is valid to 2nd anniversary of the expiry date. Dual flight anytime within 12 months of expiry.