Moe - an accde file format only protects your source code and form/report designs, it won't stop your client copying the file.
If you want to write code to expire your database after a specific period, that is your choice, but I would ensure that this was covered in the license you would get them to agree to as part of the sale. You also have to ensure that this is not easily breakable or it becomes rather pointless spending time implementing and testing it.
If your product is something that could be realistically used by other organisations, perhaps a better approach would be for you and your current client to jointly market it and sell it to them, with you sharing the profits. This way, both sides benefit.
John