How Much Advertising is Too Much?

I have a question - How much advertising is too much. We can easily identify situations where a business can benefit from more advertising, or other forms of marketing. But what about situations where it appears that the cost of advertising must be reaching diminishing returns?

This question arose in my mind related to two companies - Verizon and GEICO. Both have wonderful ad campaigns, full of features, fun, and financial information. Verizon has two major campaigns - "The Network" for its wireless business, and "The Cable Guy" for it's triple play phone, Internet and cable TV FIOS business. The ads are great, with the network geek and his army of helpers providing "the Network", and the cute FIOS guy always beating the bumbling cable guy to replace the customers cable connection with a new FIOS connection.

GEICO is even more pervasive, with four campaign themes - the gecko, the caveman, celebrity helpers, and the most recent, "money with eyes".

I think the ads are some of the best in media, and in fact I am a satisfied Verizon and GEICO customer. I have first-hand knowledge that the Verizon network is great, and that GEICO provides auto insurance at a good price with excellent service.

So I'm saying that the ads are great, and they honestly present products and services that are great. My question is - when is enough enough? These ads are pervasively presented on both TV and radio here in the northeastern USA. Do these company's know that the marginal cost of having more ads in the media is really leading to marginal sales and profits that make this investment possible, or is it possible that there are significant diminishing returns for this very high "share of voice"?

Finally, who will let these companies know that "enough is enough?". Probably not the ad agency or media buyer. Maybe the company's market research group, or brand marketing leaders (although they usually find themselves fighting for more money). The CFO has the other bias, so they are often looking to cut the marketing budget.

My sense is that a rigorous business case review would find that the optimal spending for ads on radio and TV might be less for established brands like Verizon and GEIO. What's your impression or opinion?