Is the compressor still running when it stops cooling?? If so, that is a classic sign of low refrigerant charge. The coil slowly freezes over retarding the fan flow which helps it freeze further.. If it recovers after 30 minutes or so and then goes through that cycle again, most likely needs a few ounces of refrigerant.
Are you seeing any codes on the LED readout on the control head?
If the compressor stops before it stops cooling, could be a bunch of different problems from a bad thermostat to a bad compressor and lots of stuff in between..
Remember, these should not leak so if ya recharge the refrigerant, you haven't fixed the leak.. and it will need recharging again .. time to recharge is determined by how big the leak is.. If you know when the last re-charge was, then you may have an idea of how long it will take to leak down again. One way to find very small leaks is to look for an oily patch, usually with dust stuck in it, on any joint and any other part that has refrigerant in it.
Forum member, Denise, has a lot of AC savvy .. I hope she weighs in too..