Actually just looked it up - 10 years for a FULL pension - full pension is 285k year at 62, or about 85k at age 45 on early retirement.
However - you earn partial in as little as 43 days. 43 days represents 1/4 of a credit - 40 credits is full benefit.
So say you just got called up and stuck a little more than a month - 43 days - you’d be looking at $500 a month at age 62. Not big money - but hey, social security pays what 1100? A decent bump for those guys who haven’t been near a field in 4 decades probably.