。
It's been three months since I met her.
Literal
I [subject-が] her [to-に] meet [from-てから] 3 months passed.
The pattern [verb te-form]+から means 'since [verb]ing, after [verb]ing.' Here it marks the starting event. 経つ ('to pass, to elapse [of time]') is the standard verb for time passage. An elegant way to mark duration from a significant event.