When installing the Remote Desktop Session Host role on a Windows Server, you get a 120 days grace period where you do not need to specify a license server and/or have licenses installed. After the grace period, it is no longer possible to log in with even a single session through RDP, unless you use the admin console (/admin option). Luckily, you can reset this counter by getting rid of a specific key which gets created at the start of the grace period, effectively resetting the grace period counter. This can even be done if the grace period isn't completely over.
Restart-Service TermService -Force
After restarting the service or the server, the grace period should be reset back to 120 days. You can check this through the following WMI query (run in Powershell):
(Invoke-WmiMethod -Path (gwmi -namespace root\cimv2\terminalservices -class win32_terminalservicesetting).__PATH -name GetGracePeriodDays).daysleft
There doesn't seem to be a limit to the number of times you can do this. Until Microsoft changes this behaviour, you probably can do this indefinitely. However, we are obligated to point out that if you use RDS in production environments, you are required to purchase the correct licenses!