It may depend on how much amperage your a/c unit draws. The switch is rated for a 15amp draw. But even if your unit draws less than 15amps, the duty cycle that the a/c cycles at, might be quite hard on the switch.
I had one of these fail fairly quickly with just my girlfriends flat iron being used in it.
I kind of doubt it would last long under the kind of load an air conditioner would put on it.
For an bedroom air conditioner, I would probably look for a timer designed to handle the power a/c's. The garage heater I use this for is 1200 watts and it works fine so far. But again, for an a/c unit, personally, I would lean toward getting one designed for that purpose.
They use a relay as a switch so it will support the full amount of power that a standard 120v outlet can provide. Since others have experienced failures under high load, the relay inside probably overheats after being used under heavy continuous load.
Depends on the amps/watts of the ac unit. The user guide states Continuous Duty Electrical Rating: 120V~/15A/60Hz/"1800W" So the unit has to be less than 15 amps and 1800 watts. Also I would check your breaker your using for your a/c unit to make sure it meets amperage requirements. Refer to NEC (National Electrical Code for Calculations) Per 10+ years professional and personal.
If the receptacle in question will handle the load, there is no reason to expect any problem with the Belkin device in that same receptacle. It is the wire size, the fuse/ circuit breaker size, and the type of receptacle that establishes the safety of the load.
I wouldn't take the chance. I was using it with a low-wattage heating pad, and the switch got dangerously overheated. If I hadn't been close enough to notice the smell of melting plastic, it could easily have started a fire. In my experience, this is just not a safe product.