Conversation so far:
What is Leuven for Gerardus Mercator?
Towards the end of 1534, twenty-two-year-old Mercator arrived back in Leuven to study geography, mathematics and astronomy under the guidance of Gemma Frisius.
Where is Leuven?
Leuven is Belgium

Question: How long was he in Leuven?
Self-contained rewrite: How long was Gerardus Mercator in Leuven after arriving in 1534?