Mostly , the eye number becomes stable after 18 to 21 years in females and males. Slight  variation may be possible in some people...or very few people may see a change afterwards also. Till this time ,good idea to do yearly check ...after which you can do in 2/3 years time or as per your doctors advice . Mostly, the glasses become scratched in 1 -  2 years if time, but if the condition is good, and number has not changed, can be used for longer period too.