String stretch can be a factor on a brand new bow, depending on the material the string is made of. There also can be other factors at play such as something vibrating out of place (arrow rest) or a nocking point that has slipped, etc...ect...

If you are no longer getting good tears through paper and/or your arrow flight seems a bit screwey (fishtailing, porpoising, or corkscrewing) then it is time to have things checked out again. My guess is that after this time, it will stay tuned for quite awhile (provided you don' t mess with anything).