-Normally you would say FOR 3 MONTHS e.g. I went to South Africa for three months last year.
- With a relative clause, you would use FOR THE 3 MONTHS in the main clause e.g. For the three months it took me to learn how to teach English, I rented a room near Bayswater. (I've underlined the relative clause.)
There may be other instances but I can't think of any offhand. :-D:-D