Standard retransmission rules apply. The BYE will be retransmitted until eventually it gives up and then the session is terminated locally. If the remote side never sees the BYE then the call will either continue, or it will have local policy to eventually terminate it (no RTP flowing, SIP session timers).
@jcolp timmers is a good option to deal with this issue we can use Timer F who is the maximum amount of time that a sender will wait for a non INVITE message to be acknowledged in this case the BYE request. exploring the PJSIP options I see we have
;timer_t1=500 ; Set transaction timer T1 value milliseconds (default: “500”)
;timer_b=32000 ; Set transaction timer B value milliseconds (default: “32000”)
When originally implemented there was no ability to configure such a thing in PJSIP, it’s possible that it can be done these days but noone has investigated or implemented such new functionality.