A/c runs cold then hot. Honda civic?

Honda civic 01 ex automatic VTEC

The a/c works blow cold air but after about 30mins of being on it starts blowing hot air. I noticed mostly when its hot outside. What can it be

The time doesn't make any sense. However, if during that 30 minutes you're driving on the freeway, and then after the 30 minutes, you on a residential street, it could mean that the fan for the A/C isn't working and you're about to blow out your compressor.

Start your car, turn on the A/C, open the hood on your car and look at the fans in the front. The fan on the left should be running with the A/C on. If it isn't, you need a new switch for it. If it is running, turn the A/C off while the car is still running and the fan should go off. Then, turn the A/C back on and see if it comes back on. If it doesn't, your switch, again, is faulty and needs to be replaced.

Until you can figure this out and get it fixed, stop running your A/C. A new compressor is very expensive.