A couple weeks to settle in has been my experience as well, but since I struggle to fit in more than half an hour of daily practice time, fourteen calendar days yields a maximum of 7 hours of direct string-stretching. I'm just past week 2 on a new set of High g Fremont Black Lines and they're now staying in tune, but (as advised elsewhere in UU) I jump-started the stretching process by initially tuning a full step higher than standard gCEA, then giving each string a good upward tug of around 1 inch (and holding each that way for a few seconds) just north of the saddle.
As I've mentioned in at least two other UU threads, from day one, the Fremonts have sounded, felt and looked far better than the Martin M600's they replaced. The aesthetic improvement was unexpected yet IMHO the black strings have added a degree of it's-not-a-toy-ness that is all the more surprising on an intentionally cartoonish, kit-built soprano. Because I'm a lifelong believer in "if it ain't broke, don't fix it", I'm increasingly seeing no need to try a different brand and will soon order a single Fremont wound Low G to see how much difference it may make.
Cheers!