FBinNY 

There's no "normal" rate of air loss.

It depends on several factors.

Tire width - wider tires have a higher volume to surface area ratio, so they'll lose air slower
Tire pressure - pressure is the driving force, so the loss rate is roughly proportional to pressure.
Tube material - some materials are more permeable and allow air molecules through more easily.
Tube wall thickness - thicker walls resist bleed better than thinner ones.

While the first three are pretty much fixed by rider choices based on other things factors, the last is often forgotten about and can vary significantly. Different brands have different wall thickness, so a heavier tube of the same size may be preferable. Also consider that the tube stretches to fill the tire. A smaller tube will stretch more, thinning the walls in the process.

So, one way to slow bleed is to buy the largest, heaviest tube that fits rather than a smaller one used near the upper limit of it's range.