If by "grade" you mean "prescription" mine hasn't changed in two years. Sometimes your eyes stabilize and you can keep the same prescription for a period of time, but then when you get to be forty presbyopia sets in and you start asking about bifocals.
Still, you should get your eyes checked every twelve months. So everyone raise your right hand and repeat after me. "I (state your name) do solemnly swear to get my eyes examined by a qualified eye doctor every year."

Good. Hands down. Don't you feel better now?