When do you consider a milestone met?

I've been unsure of when to mark a milestone as met or not. My daughter has been hitting all her milestones pretty early on. She's 5 months old and she can roll in both directions, sit up for several minutes at a time, put herself in the sitting positions from a laying position, and can crawl. She's even been trying to stand lately. But I'm not sure if you mark a milestone as met the first time they do it, once they start to do it more frequently or for a longer period of time, or when they've mastered it. Do I count her as being able to sit up since she only sometimes falters when she leans back too much or do I wait until she has no issues falling over? Do I go off the first time she could roll over from back to tummy (she learned tummy to back first) or once I've seen her do it a few times? When do you guys check off a milestone?