What's the ideal amount of travel for...an enduro bike?

No suspension at all. I chew up rocks and spit up gravel.
Hardtails for life.
100-mm (4 inches) of travel, front and rear
130-mm (5 inches) of travel, front and rear
150-mm (6 inches) of travel, front and rear
160-mm (6.3 inches) of travel, front and rear
170-mm (6.7 inches) of travel, front and rear
180-mm (7 inches) of travel, front and rear
I don't think about suspension travel--it's more about geometry, components and overall ride quality.