You either stop it, when YOU feel ready.
Or you stop, when SHE weans by herself.
Both my kids self-weaned.
My daughter did so at about 2.5 years old.
My son at about 1 year old.
Our Pediatrician had no problem with that.
Her kids also self weaned.
Because it was a natural weaning, I did not get engorged and my kids did not battle about it.
At sleep/nap times, this is usually the last nursing session, to end.
It really is up to you.
But, the reason for whole milk... from 1 year old until 2 or so... is because the "milk fats" in whole milk, is an essential nutrient for brain-growth.
Just keep trying different sippy's, to see which one she will take.
Or just try give her a toddler straw cup. Some kids like straws better.