Many businesses use “contact frequency” restrictions to ensure that a given lead, customer, or other contact is not drowned with calls, e-mails, or text messages; but instead considers the contact valuable and worth continuing. In my opinion, if you e-mail an address too many times, the person is likely to unsubscribe and the value of the relationship is lost.
What is surprising, in an area of heavy regulation from consumer protection and other sources, is that these frequency restrictions are largely a business decision. You should choose a conservative contact approach based on your peers, trade groups, and testing to ensure your contacts are valuable to you and welcomed by the consumer.
To my knowledge, there is only one federal or state restriction setting a specific number on call frequency. In June 2014, the Massachusetts Attorney General published regulations applicable to for-profit and occupational schools. The rules define “high pressure sales tactics” as contacting a prospective student, prior to enrollment, via telephone, text messaging, or recorded audio messaging in excess of two times in each seven day period. 940 CMR § 31.06(9).
Of course, contact is not allowed after a “do-not-call” request.
Even outside a “do-not-call” request, at a certain point repeated contact becomes harassment. That point is not defined. I, therefore, recommend using a conservative industry standard as a starting point, then evaluating feedback from your representatives, customers, and clients to reach a solution best for all.
 See AG Coakley Announces Finalization of New For-Profit and Occupational School Regulations, Attorney General of Massachusetts (June 25, 2014), http://www.mass.gov/ago/news-and-updates/press-releases/2014/2014-06-25-for-profit-regulations.html.