Tough question to answer, many factors to consider. Your market size, demographics, number of competitors, growth rate desired, age of your firm, previous advertising history (branding vs. ads intended to get immediate response/jobs), economic situation in your area... etc.
Our advertising %'s skyrocketed this year and last for many reasons, having to lower prices due to competing firms dropping theirs, higher advertising costs, competing against more companies advertising, higher fuel costs, mild winter, record drought,.... the list goes on.
To keep 4 going I'll swallow roughly 20% this year.
But my market isn't your market. I made friends with other "elective" service type firms in my area. We for years have shared market data in a non-competitive way and it has been a great source of insight overall.