When her udders start filling, you could be anywhere from 2 weeks to 4-6 weeks away from foaling.
Sierra started filling up about 3 weeks out. By the last week, it stayed large and firm. When the milk turned white she was I believe 3 days out, when it turned a cream color (colostrum) she foaled the same night.
When she gets real close, wrap her tail so that it doesn't get gross. I wasn't able to wrap Dixie's (a palomino QH I had) because I didn't know when her exact due date was and she just dropped right out in the pasture without any signs of pending foaling.
Her tail was really disgusting after that, took a long time to wash it all out. Not to mention she was nearly totally "wild" when I got her.
Dixie didn't follow the "textbook" and as soon as she foaled, she had a nice large bag out of nowhere.
Every mare is different, and I hope she lets you see her foaling! So far Ive been lucky enough to be able to watch every foal be born here.