I was wondering if monsters in dungeons or in field respawn after an amount of time? Could be great for loots and XP farming.
No, monsters do not respawn.
If you want more monsters, you can leave the game and re-enter to respawn them.
Monsters do respawn on the ps3 version. If you clear out one "area" then go clean out 4 more areas you can come back to the area you started in and the monsters will have repopulated. I dont think this works on pc but ive done it hundreds of times on the ps3 especially to farm ubers.
An area would be defined best as a level or anywherethat u enter a portal and the leave through another portal or dungean teleporter.