Previously, Dot TK free domains had the requirement that 25 visitors per 90 days should visit the domain in order for it not to expire.
In early 2012, we removed this requirement, so your domain will remain active now regardless of the number of visitors.
You will need to renew the domain prior to the expiration date, though. The longest possible registration period is one year. You will receive an email when your domain is about to expire.