It's best to make sure the fry are fully developed...If you strip too early and they still have egg sacs, you could risk losing some of them.
Day 18 seems to be the magic number for me!
Fry will develop quicker in warmer temperatures, my tank sit between 78-80 at all times, so this is also a factor to look into in determining your strip date.
You can also use the turkey baster method for stripping small females...Works great with young demasoni!
Kim