Is there a time limit on how long completed projects can remain in Photos?

Is there a time limit on how long completed projects can remain in Photos?

No, there is no time limit to completed projects. Completed projects can remain in your Photos Library, as long as you want to keep them, just in case you want to order the book or calendar again.

If you upgrade to newer versions of Photos, the themes may change however, and then book or calendar may also change slightly.

A project can remain in your Photos library as long as there is a library. There's no time limit for projects.