it depends on your setup, my commuter runs skinny tires at high pressure ~100psi and those tires get aired up once a week to maintain pressure. My mountain bike runs significantly lower pressure ~35-45 psi and I rarely air those up, unless I have a flat.

It may be just normal leakage, you could just keep airing it up every week, or if that is too much trouble change the tube. Its an easy change depending on how old your tires are.