Thanks for your question. If you purchased a "perpetual" license for the product, you can use the version you purchased as long as you want.
This information can be found on SupportNet without having to log in. Here is the link: https://supportnet.mentor.com/about/policies/authorization_code_duration_policy.cfm
And here is the published policy:
Perpetual Licenses at Declined Sites: A declined site is a site without any products on current support contract. Perpetual licenses at declined sites will have Authorization Codes generated at the last supported version with dates that extend for five years.