How long should I wait for my gas oven to heat up?
You wait until it's hot... if it doesn't have a built-in indicator, buy an inexpensive oven thermometer - they can either sit on a shelf or hang from one, usually. And see when it gets to the right temperature. 

If you're dealing with an old and inexpensive oven, it may be miscalibrated anyway, so you'll want the thermometer to tell you if setting it to 350F is actually making it 350F in the first place... this will allow you to adjust if necessary (may need to adjust either up or down, depending).

How long it takes will depend on the oven's efficiency and how well it's insulated, so it's impossible for us to guess. It could be anything from 5 to 30 minutes, which is why the thermometer is necessary.

If you don't want to buy a thermometer (though I strongly suggest that you do), I'd wait at least 20 minutes. There's nothing wrong with leaving your oven preheating "too" long.