My tire psi levels are at 30 for each tire. I drive a GMC Terrain. When should I start to think about putting air in them?
Check the pressure with a pressure gauge regularly, when it drops 2 psi below the recommended pressure, top them up. Always measure the pressure when the tires are cold (car's been sitting for a few hours in the shade).
There's a sticker, either in the glove box or on the driver door pillar that says what the recommended pressure is. Keep it within 2 psi.
As you ask specific timing for filling air into tyre as per your application but generally preferable every week. Check your tire pressure and ,if required, fill it in cold conditions common in the morning when first starting a trip.