Borderlands – What missions do not give unique loot

To my knowledge only white loot despawns (this includes things like ammunition) and while I don't know how long it takes specifically it is possible to tell by looking at it. The item steadily shrinks in size until it basically disappears. So yes, it does seem to be based on time.

I'm fairly sure items rarer than white (green, blue, purple, E-tech, orange, obviously) don't despawn. I've left them lying in open chests before and come back ten, 15 minutes later and still seen them lying there. I'm not certain about them being on the ground though, if you take them from the chest then drop them again they may have exactly the same "shrinking away into nothingness" time as white items.

In playthrough 1, outside of the DLC, there's no such thing as scaling. Enemy levels are determined solely by the area they're in and your mission (certain missions effectively "level up" areas, notably Sanctuary and The Dust). You can easily check the level of an area by checking out the vending machines in the area, they sell items scaled to the level.

DLC in playthrough 1 scales to your level...ish. It maxes out at level 30. I played the DLC as a level 34 character and was disappointed to see everything was at level 30 in playthrough 1. Not sure if it has a minimum level it scales to.

Playthrough 2 works the same way until you beat the Final Boss; then everything in that playthrough scales to your level (except mission rewards for missions picked up before the final boss).

Playthrough 3 works the same was as Playthrough 2 after the final boss: Everything scales to your level, period. In team play, it will always be scaled to the highest level player's level. And you have to be at least level 50 to unlock it, so enemies are effectively level 50-61 at all times. Though if you take a mission before leveling up, the mission will still stay at that lower level, as always.

Basically the game works like a standard RPG without level scaling until you A) beat the game twice (because you should be maxed out anyway) or B) enter a DLC (because you can do that at any time so a preset level would be a really bad idea)