لقد مضى عشر سنوات منذ آخر مرة رأيتك فيها.
Explanation:
منذ (since) is the most appropriate preposition to use in this context because it indicates a point in time in the past from which a duration continues up to the present or another point in time.
عشر سنوات (ten years) is the duration of time that has passed since you last saw the person.
آخر مرة رأيتك فيها (the last time I saw you) is the specific point in time that you are referring to.
Here are some other examples of how to use "منذ" in a sentence:
لقد مضى أسبوع منذ آخر مرة ذهبت فيها إلى السينما. (It has been a week since I last went to the movies.)
لقد مضى شهر منذ أن بدأت هذا المشروع. (It has been a month since I started this project.)
لقد مضى عام منذ أن انتقلت إلى مدينة جديدة. (It has been a year since I moved to a new city.)
I hope this helps! Let me know if you have any other questions.