Whenever a bug despawns or is captured a new one spawns somewhere on the server within 10 seconds. There is a limit to the number of bugs that can be on the overworld at any one time. For every bug that is in an unreachable area, that is one less than can be captured. Having a long despawn time, let's say 6 or 24 hours would make it so you would basically never see a bug despawn, but they wouldn't all end up in unreachable areas after a few weeks.