i currently have a tolerance (amount of time i can play without losing lip grip) of about 40 minutes.
is there any way except for time to make it more?

you need to gradually build it up - practice 5 extra minutes a day.

alex

the great John Mack taught me to play for 30min., take a break 30min., play for 30min.,etc. He told me that's how he got ready for recitals.

John Mack was my oboe teacher's teacher after Mariotti passed away.

I know that practicing is like excercise. You don't want to do too much at first, but as you improve, gradually increase length and difficulty of practice.