Not at all; if you have an active relationship with your antivirus provider and give them consent—because of course what you want is for them to send you updates every month, so you give them the consent, saying “send me updates every month”—and they do that, this can continue until you decide to withdraw your consent.
In talking about the 18 months, or the six months, we're talking about instances in which you haven't provided consent; instead, what we are doing is implying that you would agree to having this sent, even though you never really said that you would. For something like antivirus software, though, surely the whole point of it is having regular updates as new viruses emerge. Surely the consumer is going to consent to those regular e-mails.