Hmm...so they met in May and the FNIT happened when? June? 
Hi Kitty,
I would have replied to your question earlier but I've been away in Europe for the last three weeks and only got back last night. The time between Ennis and Jack first meeting and FNIT is a lunar month or the time between the appearance of one full moon and the next. So:
May 8. Ennis and Jack drive the sheep up Brokeback Mountain. Ennis remains in camp but Jack goes up to his little pup tent to look after the pastured sheep. When Jack looks down on Ennis's camp below, a full moon is shining.
June 7. FNIT takes place. A full moon is again seen. So almost a calendar month has elapsed between when they first met and FNIT.
As for how long they are on Brokeback before being called down by Aguirre, the answer is about three and a half months. They first meet outside Aguirre's office on May 7. The first snow occurs on August 13 and Aguirre orders them down "the next week". So presumably they make the trek back down the mountain around Aug 17 or 18.