you can call a method on startup of your application that checks the installation date. if it's more than the required days, don't allow to proceed to user.
How to get installation date, you may refer to following link:

http://social.msdn.microsoft.com/Forums/windows/en-US/001bdbdd-a0c9-4356-b32c-65c819f26a6b/how-to-make-my-project-becomes-30-days-trail?forum=winforms