[Marsel]

Welcome to the forum, shari-aree.
I'm not avalon but I'll be delighted to answer your question.

Arthur and Catherine's marriage was arranged when both were two years old (as part of the Treaty of Medina del Campo).
They were married by proxy in 1499 (both were 14 at the time).
Their actual marriage took place in November of 1501 (Arthur was 15, Catherine - 16) at St. Paul's Cathedral.
Arthur died in April 1502.

If one counts their marriage from May 1499 (the day of their marriage by proxy), then they were married for 2 years and 11 months.
If we starts the count from the date of their actual marriage (which took place days after they had met each other for the first time), then their marriage lasted only 4 months 15 days.