Those are a month and a half old? May I ask what you used for lighting?... Those babies should be pushing 18 inches tall or better at that age....you would be lucky to pull a 1/4 oz. off of them combined at that size....
assuming that's outside it really depends on how cold it gets in autumn where you live. If it's like our climate here in michigan I'd say from a quarter to a half because it's very late in the year.