Bike tires need air every ride...is this normal?
If you're putting in 5 or maybe even 10 PSI every week, that's not too much out of line; it's normal for tires to lose a little bit of pressure over time. 

If your tires are sinking down to much more than that, you may have slow leaks. If both tires are decreasing by the same amount, perhaps you have older tubes that need replacing. If one tire is doing this but not the other, you may have a very slow leak. Try replacing the tube and see if that fixes the problem.