If a girl started developing boobs around the age of 10, what age is she most likely to get her period?

Boobs don't = period. Breast development in both genders is often one of the first signs of development. A better harbinger of the first period is the production of cervical mucous. This is clear, slimy non-smelly discharge from the vagina caused by maturing cervical glands. You'll see the discharge in your underwear. This usually happens about 6 months before the period starts.
Within 3 years. As a rule, I would want to evaluate a young lady if she did not progress to menses within 3 years of initial breast budding.I find looking at a growth chart can help. I look at the probable point is the age the girl will be when she is 2.5-3 inches away from her projected final height predicts this within a few months. Clear discharge is common for the weeks before they start.