On an EASA Part-MED certificate it usually says on the front. If not I believe it depends on when your medical was done. If you did your medical the day before your 40th birthday then you would get the 12 month validity period (as you were 39 when you had the examination).
In your case the privileges last until the expiry of that medical certificate, Then at your next medical (when you're 40) you will only have 6 month's validity for SP ops.