When I changed all of my tyres over to new ones the light came on after about 10 minutes of driving.
Best thing to do is make sure all of the tyre pressures are the same, then with the engine off and ignition in the second position (i.e. all of the dash lights on etc) press and hold the button situated under the handbrake.
Whilst still holding the button start the engine. The light on the dash should remain lit, if so let go of the button and start driving after a few seconds it should go out.
The next time it comes on, should mean that one of your tyres has deflated slightly. Check the pressures then re-set as above.