I'm going to clarify that situation by citing an example. I give my e-mail address to a person and tell him to contact me in future. That may be in three, five or six years; there's no time limit on express consent. If the person to whom I give my e-mail address forgets to ask for my express consent, he has 18 months to do so by following up by e-mail.
We took the 18 months from the other bill—the Do not call list. I consider a period of a year and a half quite enough time to follow up with a client. A legitimate business should do so within four, five or six weeks. If express consent is obtained, it is valid for an indefinite period of time, but you have to set a date for implied consent.