What the *checks where thread is posted* frickin' Corpsman said.
I enlisted in the Navy through their nuclear propulsion program. When I graduated boot camp I was an E-3 and got back pay for it for the time I spent in boot camp. (Everybody is an E-1 in boot camp.)
When I completed ET-A School a few months earlier, I was promoted to E-4 (Third Class Petty Officer) as part of the program. So in less than a year, I made E-4. If I recall, about 8 months, give or take.
When I took the first E-5 exam the following year (a soon as I met the time in rate requirements), I passed with a high enough score to be advanced to E-5 (Second Class). Less than 2 years in and with no time at sea.
Likewise, I took the first E-6 exam I was eligible for and made it first time up. I wanna say just under 5 years.
It's a lot like the civilian world in many aspects. If you've for the education and/or skill sets that are more highly prized, you'll go further and faster than those who do not. Or if your in a field that's not a desired by others and had a hard time getting and keeping people.
Now, it took me quite a while before I made Chief. The requirements are different and I went through quite a lot (personally and professionally) before I "got it"...but when I DID get it, it didn't take long afterwards. Still, it was just before my 17 year point when my anchors were pointed on.