Even before you get to burnout you get saturation where more advertising yields lower returns. There is a whole industry around advertising analytics and helping advertisers figure out how to allocate their budgets for maximum penetration without wasting money on saturated audiences.

If you're interested in some advertising theory, give Media Planning: A Practical Guide by Jim Surmanek[0] a shot. It's short and pretty accessible.

[0] - http://amzn.to/1Q7mGVe