grammar - "30 year" vs. "30 years"

Reading a report online, I read something like this.

during his 30-year rule.

Is it 30 year or 30 years?

Answer

"30 year" would be more conventional. "He ruled for 30 years, his rule lasted 30 years. it was a 30 year rule." I think the reasoning is that 30 year is taking on the role of an adjective in the final sentence, and that's the relevant difference. Note, however, the "Hundred Years War". In this construction it's possible that Hundred Years is acting as a proper noun rather than an adjective, hence the inconsistency.