The flowering stretch in most cannabis plants lasts about 2 weeks. Sativas will rise more than indicas in that time, but it lasts about the same amount of time for both subspecies. The only plant I've ever seen go longer than 2 weeks is Dr Grinspoon - that's a proper heirloom sativa and it didn't stop stretching for nearly 4 weeks!
As you say, it's completely dependent on genetics, but I would estimate 50-100% stretch with indicas, and 75-150% stretch in sativas. Hope that helps!