Have a look at http://www.granite.ab.ca/access/demo.htm for some ideas.

In addition to the link Joan offered, here are a few more:

http://members.shaw.ca/glenk/access2000.html
Look for the downloads:
"Updated have Database Expire After 30 Days Of Use"

"Updated have Database Expire After 30 Days Of First Being Opened."

MVP Tony Toews' thoughts on demos/expiration coding:
http://www.granite.ab.ca/access/demo.htm

Peter De Baets has a product called KeyedAccess that will do this I believe:
http://www.peterssoftware.com/ka.htm

Paul Overway has something similar here:
http://www.logico-solutions.com/
--Go to Add-ins/Extras area and look for "Logico Solutions MDB Lock"

Also try here:
http://www.rogersaccesslibrary.com/Otherdownload.asp?SampleName='LicenseLock'

Good luck,