In the US, probably indefinitely. There’s not communication between municipalities to actually track down someone killing local prisoners. Depending on where it happened, it might not even be noticeable among the numbers that die in prison already. The single most improbable part of death note is the initial speed in connecting widespread events to a hyper localized area.

The police also have a terrible solve rate on murder nationwide. Serial killers have decade plus long active streaks while physically killing and hiding bodies. There’s several that never were caught.