From the 5th through 1st centuries BC — 400 years — Rome was at war roughly 90% of the time. 10 to 30% of its adult male population was enlisted in the armed forces. Did other cultures commit so much of their population to war for such long periods? How unique was the Roman war machine?

Where did you get 30% from? Brunt's "low count" (which refers to the register numbers) has about 10% of the male population mobilized.

Lo Cascio more recently proposed a "high count" (again referring to the register numbers) that, if I remember rightly, puts Rome's mobilization at around 7%. I think he also claims that's a level of mobilization not achieved again until the total war of the French Revolution. 30% of the male population under arms is downright unthinkable.

The one exception might possibly be in the aftermath of Cannae, when Rome was desperate enough to enroll a legion of slaves.

If there is some confusion in this question, it may be based in the fact that each Roman legion was accompanied by either one or two legions of socii. The base of manpower Rome could draw on was far larger than its own population, especially after Punic 2. Rome might be able to field armies far larger than its population would allow on its own, because it augmented its own armies with these Italian troops. In that case, it might be true that Roman armies were as large as 30% of Rome's population, but I still would be shocked if the numbers worked out that way.