What Car? investigates: tyre safety - Conclusion
A slow puncture, poorly sealed tyre or porous alloy wheel will allow air to escape faster, and while the tyre might not look flat for months, you'll be throwing away money and using a potential death trap as your daily transport.
Check your pressures weekly, and always when the tyres are cold. Even a short trip to the local petrol station can warm the tyres significantly.
If you are travelling at speed for long distances or carrying a heavy load, adjust your tyre pressures before you set off.
Check your car's handbook for the recommended PSIs.