One extreme or the other: Morrowind's named bandits didn't respawn.....ever; Oblivion's bandits respawned in 3 days.....inevitably. A "chance" to repawn after some amount of time would have been great, especially if it tracked the time since your last visit and varied the odds accordingly. That would make a constantly re-visited site relatively safe (so you could use it as a "home"), while one that you've ignored for the last three weeks might be occupied again, and one that's been ignored for 6 months is likely to be.
Managed to do a proof of concept mod that produced respawning dungeons once the original named NPCs were dead, and also made it so that they would only respawn if you didn't keep visiting the dungeon.
Only downside was that it requires a script on every NPC that is in every dungeon, which is a little tedius, but not impossible. Also, new spawns would only appear at the location the original NPC was killed at.