What breed? from what I've seen, in general, dairy heifers tend to bag up months before calving, beef heifers tend to start closer to the due date. Jerseys seem to be the worst in my limited experience with them; I've seen open Jersey heifers that were they a Holstein, I would have called them 7-8 months bred by their udder.
I think the earliest I've had was one AI-bred Holstein heifer that started bagging up around 4-4.5 months bred, looked like she was 7+ months.