OLMP took me exactly 18 months (took a term off for newborn), so could be done slightly faster if necessary, but I worked my butt off and my wife was incredibly supportive.
Did the joint warfare concentration, which was the standard at the time (non competitive). Was an interesting experience. Most classes had about 15 peeps. If you were lucky there'd be 4 operators in a class at once (alarming number of civilians, then ARC, very few AD). So maybe talking 2 individuals with any joint experience per course. What that meant for most classes was that you had people with absolutely no knowledge of operations or even other services attempting to be experts on joint warfare. Sometime that was pretty laughable (ok, most of the time). I'm afraid I did an awful lot of mansplaining. You figured out pretty quick who was worth engaging with online, and who was stringing together sentences solely from the week's learning objectives.
There was some academic value to the actual coursework. The research classes were actually excellent, and would have worked wonders when I knocked out my real master's a decade ago. Several of the other courses were highly entertaining and informative, but that was primarily a factory of the instructors. Some were clearly collecting a check (grade depends entirely on comment output, rather than content). Others were actually there to help facilitate learning...those were the best.
In retrospect, would recommend if your life situation accommodates. Yes, it's a free Masters, which isn't nothing, but it also takes WORK, which may not be required to accomplish whatever your big blue dreams are.
Hit me up if you have any specific questions.