I had 3mm removed from my upper lip. Can I gain it back by stretching it? How often and how long should I stretch it?

Natural healing and the effects of gravity will cause some stretching of the lip shortening effect. (relapse) It will probably not be the full 3mms but likely half of it. By some some gradual streching between 3 weeks and 3 months after surgery, you may be able to exceed the amount of normal relapse that usually occurs from a subnasal lip lift procedure.