A yearly CERF exam is required to be CERF certified. Some parent clubs do allow every 2 years (For cHIC) but yours is not one of them. However no matter what the parent club states CERF themselves says yearly for all breeds to be certified.
There are currently 635 Mastiffs who not only are CERF'd yearly, but have their hips, hearts, elbows and are PI. So some are doing all the required testing for your breed.
Many breeders will only redo CERF when they plan on breeding so this could mean skipping a year or two, thats not uncommon.
I see they now have a PRA DNA test for them what a great thing for the breed!!