Fuel usage will depend on the make and model of the car, but switching off the air con for long periods will cause the seals on all cars to dry out and allow all of the refrigerant gas to escape. A regas will cost between £100 - £250 and fixing the seals will easily add another £100 - £200 on top of that (otherwise the gas will escape, again).

Many AC compressors seizures are caused by the fact the unit has been switched off for long periods. And a new compressor will cost £500+.

Add in the fact that your car's heating and ventilation system is designed to operate with the AC on 100 per cent of the time and I would argue that it's unwise to switch it off to save a few pounds on your fuel costs.