There are certain laws that exist in the universe that should be considered when looking at your business.
One of these is "The Law of Inertia".
Inertia is defined as "the tendancy of all objects and matter in the universe to stay still if still, or, if moving, to go on moving in the same direction."
Think about this in relation to your advertising.
If you haven't been doing any advertising -- it takes a little while to get it moving. You have to nudge it along until it picks up speed. Once it is going pretty good, you start to see results. And, if you were to stop you ad campaign (something that I would NOT recoimmend), the momentum would cause you to still see results for a while after that. Momentum is a wonderful thing ... but it doesn't come cheaply.
It is much easier to keep the ball rolling once you have it in motion.