I ride my bike maybe once a week at most. I need to add air to the tires every time to bring it up to the right pressure. (I ride a mountain bike and I put about 45 - 50 PSI's into the tires.) 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.
The posh 'extra comfy', low-rolling resistance 'latex' tubes that cost twice that of normal tubes can deflate quicker than is desirable.
Valves can also be a problem, typically from getting chalk dust particles inside the valve innards. This can be the case with both valve types, however, with Presta you can mitigate against this by ensuring the valves are done up tight.
Remember that rubber perishes when in contact with the elements, to minimise the chance of this make sure the valve caps are present.
As noted, if one tyre deflates quicker than the other then it is likely that you have a slow puncture. This puncture may not necessarily be visible to the naked eye, holes can be a 'normal' part of the manufacturing process or they can be introduced around the valve seat.
Different tyres deflate over time at different rates. This is proportional to the tyre volume, e.g. a 20" x 1.5" tyre will deflate quicker than a 26" x 2.3" tyre. 5-10 PSI is borderline acceptable, even if accounting for air lost connecting up your track pump. You should only need to attend to tyre pressure every fortnight.
High-pressure,low-volume tires like typical roadster tires often need to be checked daily.
They might only need a few pounds pressure to bring up to normal, but it's best to check.
Lower-pressure higher-volume tires like typical MTB tires seem more forgiving, but in maintaining our department fleet of 25 patrol bikes, I find a minimum of weekly checking is necessary. It's next to impossible to tell without actually putting a gauge on the tire; the thing may feel quite firm and still be 20 pounds under pressure.