We've seen drifting of about 10% only after one year of operation. The element in which the device is located is extremely harsh. What could be causing the drifting? ....we are looking to switch pressure switch vendors. How often do you calibrate your pressure switches? and what is the typical drift seen with the as found/as left percentage?

There are some more things which we could discuss like:
The service/ application in which the switches are being used for e.g boiler furnace pressure & for alarm or tripping
The type of switches,make model,
set point
The drift in the switch may be due to a number of reasons:
1) Vibration in the impulse line
2) moisture ingress
3) loose conections
4) Improper fittings.
The calibration frequency varies from application to your maintenance practices.
In the case of boiler during IBR shutdowns in India which is to be done these things are checked.
In other cases like gas/steam turbines the frequency may vary according to the HGPI/Major shutdown.
But whatever the frequency within one year 10% drift is not allowable.
We have seen switches after 15-20 years giving accurate results .
is this the problem in a single switch or is it affectinga whole lot then in that case you may need to conatct the vendor.
If by meaning of harsh servicxe you mean acidic/caustic application then also you have specific switches by all vendors for various applications/services.