700c x 32s at 50 PSI or so, I've gone 5 weeks and probably could have gone longer. I'm usually down from 50 to 30 or so in 5 weeks.

On my new bike with 700c x 23 at 110 PSI, if I don't pump them up every week, if I hit a rock or rut I WILL get a pinch flat (had it happen twice this spring already). In a week I'll be down from 110 to probably 75 or so. I really should be pumping them up every couple of days. That's why I bought a good pump along with the bike, after all (bought a Joe Blow).

The smaller the tire, the more of a difference losing each CC of air makes (in terms of PSI). Also the higher the pressure, the faster the air will make its way out through the rubber or the valve stem (though a Presta valve helps a little there). So narrow, high pressure tires need to be checked FAR more often than wide, low pressure ones.