Your licensing for the PSCAD Free Edition does not expire. The Free Edition may be used and evaluated for as long as you like.
What does expire is the certificate "checkout period", which is set to 28 days. This checkout period was developed to allow customers to run PSCAD in locations with no Internet access.
Internet access is required to activate the certificate, initially.
In order to restart the checkout period to the maximum number of days, simply launch the Free Edition with Internet connection.
For more information on certificate licensing, please refer to this article.