If the client dies, the server frees the license, unless the client's whole system crashes.
Assuming communication is TCP, the license is automatically freed immediately. If communications are UDP, then the license is freed after the UDP timeout, which is set by each vendor, but defaults to 45 minutes. UDP communications are normally only set by the end-user, so TCP should be assumed. If the whole system crashes, then the license is not freed, and you should use lmremove to free the license.