Cosign with the above: depends on your hair and what it can take. I recently learned that 4 months was entirely too long for my hair, so I won't stretch past 12-13 weeks again. But ladies like sylver can go 6 months to a year and do just fine.
My suggestion would be not to set a time frame... just add a week to your normal stretch time, and assess your hair weekly thereafter. If it seems to be okay, add another week. If you start to notice shedding/breakage/etc, then it may be time to stop stretching. Listen to your hair, don't tell it what to do.