Hi Lawrence,
The Captivate source project file (.cptx) will never expire.
If you had Captivate 8 trial earlier and you created a project on that machine and published the output, then output will expire after 30 days of publish.
Once you have purchased the software, you need to reopen that .cptx file in purchased copy of Captivate and publish it again from there and that will never expire.
Thanks.