FWIW...from a Patent Attorney:

"A patent remains enforceable during reexamination, so can collect on
past infringement if the claims are unchanged. Collection of licensing
fees depend on the terms of the licensing agreement. The licensing
agreement typically ends when the patent expires, and the technology
falls into the public domain."

Also, as explained by D2006, we can collect as far back as the date the potential infringer was put on notice, or according to the PA, "notice by marking the product".

Good luck everyone!