This is a pernicious myth.
There is no such thing as abandonment of copyright.
You are thinking of trademark.
Your copyright cannot be lost through a failure to enforce. Ever.
The difference is that trademark protects a distinctive mark or word from misleading uses in the stream of commerce. If a mark loses its distinctiveness, a court might rule that it has become generic (this happened to "cola" for example, which is why we have both Pepsi Cola and Coca-Cola). This happens very, very rarely, though it does happen.
Copyright, on the other hand, protects creative expression (not uses in commerce) for the life of the author, plus 70 years (or a total of 90 years for works-made-for-hire for corporations).
Copyright has many exemptions, including those that a court might call fair use, as well as incidental uses (called "de minimus" uses), through which you are allowed to make and adapt works without permission.
Trademark doesn't have fair use per se, but it only applies to commercial use, and its exclusivity is limited to confusing uses. That means that trademark only allows you to prevent people from using marks in a way that is likely to confuse a "naive consumer" about the origin of goods or services.
It's really important not to repeat the myth that unenforced copyrights lapse. This is used as a catch-all excuse for otherwise inexcusable bullying. it also frightens people into threatening other people due to fear of loss of their copyrights.