Samu Salminen's listing on the NJ Devils Reserve list says his rights expire June 1, 2025.
He plays for the Jokerit junior team, which plays in Finnish league, but parent club plays in KHL.
He is committed to the University of Denver, so will likely stay in juniors until he enrolls in Fall of 2022.

So are his rights treated like a KHL player? Or like a Liiga player? Or a college player?

KHL Player - indefinite expiry
Liiga junior player - 4 years from draft, so June 1, 2025
College Player - August 15th after graduation, so likely August 15, 2026.