Jake,
From my experince, females "typically" develop at about 3-4 mm a week after they reach 20. However, that is typical, and can vary greatly from female to female. 10mm is a tough number to gauge, as they can sit between 8 and 14 for a long time and never do anything. Hope that helped some. If she is developing well, I would say about 6 weeks, but don't hold me to that.