When To Check Tire Pressure
Check inflation pressure (including the spare) at least once a month and before every long trip. Tires must be checked when they are cold (before they have run a mile). If you must drive over a mile for air, measure and record the underinflation amount of each tire. Upon arriving at the service station, measure each tire's inflation again and if the pressure has increased, adjust the amount of additional air pressure needed. For example, if cold pressure should be 35PSI, cold pressure was 28PSI, and current pressure is 33PSI, you should inflate the warm tires to 40PSI and recheck them again when cold.
http://toyotire.eu/page/index/sef/proxes-cf1/identifier/proper-inflation