'99 Durango - A/C won't stay cold

On my '99 Durango the A/C has gone hot. If we add more coolant it stays cold for about a day and then gradually gets warmer and warmer until its just hot air again.

I'm trying to get the vehicle into optimum trade-in condition and I'm new to learning to repair my own vehicles, so ANY suggestions as to what the problem might be would be GREATLY appreciated.

Thanks much.