Saturation is one of the core advertising fundamentals and a key driver when determining your budget. The key is to match your budget to the size of your audience. Spread yourself too thin and you likely won't see any impact.
Here are some general spend ranges based on the size of the area you're looking to target. Remember, you're better off advertising to a smaller area with the right budget than a larger area with too little budget. Think of targeting a few highly traveled routes vs attempting to canvas an entire city.
To learn more about how to determine your overall advertising budget click here