Borderlands – How does the elemental DoT of two weapons of the same element stack

The length depends on the elemental damage type. I found these out by testing with Target Practice in Marcus' shop in Sanctuary.

• Fire: 5 seconds
• Shock: 2 seconds
• Corrosive: 8 seconds
• Slag: 8 seconds

This means a Corrosive gun with the same damage/second as a shock gun will deal much more damage than the shock gun if you let the effects linger. It also lets you stock a lot more Corrosive/Fire effects on a single target than you can Shock effects, but Shock effects will usually drain a shield so fast you can switch to another weapon to deal the health/armor damage instead.

Note that certain enemies resist these damage over time effects entirely, and bosses may take elemental damage for much briefer periods of time; for instance Terramorphous can only be Slagged rarely, and when it worked Slag only works for 1 second. Most normal enemies don't have shortened effect times however.

The health bar has to be yellow for corrosive to be extra effective. It still works normally on most normal health bars. Fire is best against most normal health bars (except the ones of fire creatures as they will resist). Shock is great against shields, especially useful in the second playthrough when much more things have shields.

In general you should be able to judge what is effective, what is resisted and what is doing normal damage just by looking at the damage numbers floating around enemies.