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I can't remember the source of this information, but the rule-of-thumb for effective frequency is supposed to be 7 times exposure for a single ad (the same ad) within the same media within a campaign cycle. Having said that, there are many other variables-circulation, readership, target audience/reach, frequency distribution and so on.
The most critical component is the creativity of the ad because with a memorable ad, one can break through the media clutter and generate significant impact even with a few ads, in fact even with a single ad. This is why advertisers with limited budget sometimes resort to outrageous ads so as to generate good recall and memorability, sometimes at the expense of its image.
The rule, of course, would be different for a B2B advertising since the approach would be more rational, rather than emotional, and the media would be much more targeted, i.e. trade media, with high concentration of readers who would naturally be interested in the ad content so long as the message is clear and concise, regardless of any creativity angles. One should also be mindful of that old expression: "if it sells, it must be creative."