Damage Reduction questions

[Revweiler]

So, I'm homebrewing some stuff up for Pathfinder to make Monks a little more fun.

On to the questions: How much DR/special material would a character need to for it to matter without it being broken?

For that matter, how much more DR would a Barbarian need for his DR to be useful?

[Keneth]

An invulnerable rager gets DR/- equal to half his level. This is a significant number but not really a lot. Having DR/material equal to level would be a decent ability, slightly more so if the material is adamantine.

[DeltaEmil]

Damage Reduction 15 or 20 ought to be okay at level 20. DR 20 if you can bypass it somehow, and DR 15/- if you can't bypass it.
The 20 should have two necessary components to bypass it. Like for example DR 20/magic and chaotic, or DR 20/silver and good. Stuff like that.
If the Damage Reduction can be bypassed by one component, it might be okay to increase the DR to 25 (DR 25/bludgeoning). If there are multiple components that could bypass it, 30 might be okay (DR 30/bludgeoning or chaotic).