Wandering trader sometimes despawns after 20 minutes, as expected, but at other times will linger in the same spot for days without despawning. Current trader spawned with llamas on top of a bell I placed in a village created/populated in survival, but has not despawned after several days of play (both in-game days and real time days.) Left the area for an extended journey for a couple days and came back. Merchant was still in the same place.