The a/c in my truck blows out cold air for about 20mins then it just blows. Any idea?

the a/c in my truck blows out cold air for about 20mins then it just blows. any idea?
Can you hear the A/C cycling (clicking)?
The system charge may be too low to allow compressor operation. Or the cycling switch or clutch relay may be sticking in the OFF position.
Check all the fuses and make certain they aren't open.
Look in the relay center for the A/C relay and while attempting to run the system, tap on the relay to see if it kicks on.
Let us know.

Can you hear the A/C cycling (clicking)?
The system charge may be too low to allow compressor operation. Or the cycling switch or clutch relay may be sticking in the OFF position.
Check all the fuses and make certain they aren't open.
Look in the relay center for the A/C relay and while attempting to run the system, tap on the relay to see if it kicks on.
Let us know.

Thanks, I will try it. I will let you know what I find.