When is the best time to start working on a masters degree?

Now. There's no perfect time to do it. And it will take longer than you think. I took two years off between Bachelors and Masters, and the curve of getting back into school was profound. As far as schools and courses of study: start by asking yourself, "What do I want to do?" Then, work your way out from there.

I'd recommend that you subtly restructure your thinking about a graduate degree. First decide what you want to achieve down the road (20-25 years), and then use that information to decide what type of graduate degree best helps you to accomplish that goal. In other words, your long-term goals should drive your short-term decisions. Then, look for the graduate schooling options that can fit within your short-term constraints.

If your goal is to be a career military officer (with no follow-on civilian career), then pretty much any graduate degree will work. You just need to check the box. Some degrees will help you be more effective in the various jobs throughout your military career, such as management-related degrees, technical degrees (if you pursue a functional area), etc., so that may adjust your preferences somewhat. You just need to decide where/how to fit those options into your military career path. Many options exist for executive, online or hybrid programs that will enable you to fit your schooling around your military job. You can chip away at it as early as possible, but don't go so fast that you undermine success in the job, and you might want to delay the heavy lifting until after command. A successful command will set you up for many things later on in your career. An unsuccessful command could quickly undermine your career.

If your goal is to have a long-term civilian career after your military life (meaning more than a handful of short-term jobs), then you may need to take a different approach. You can still consider the executive, hybrid, and online programs, but you should branch out to look at some of the civilian schooling options that the military provides (where they send you to a traditional program). I went to Syracuse to get my MBA because I was focused on my post-military career, much more so than how I'd use that degree within the military. The military had a program where they sent me (and my family) to Syracuse to focus full-time on my graduate studies. I have a good friend who was sent to Georgia Tech. Both of us eventually went back to school again (under another military program) for our PhDs while still on active duty.

The bottom line is that I'd recommend pursuing a graduate degree more strategically, with letting your long-term goals help you select the appropriate discipline, academic institution, and mode of delivery for that graduate degree. Regardless, best of luck to you!

I started my Masters as soon as I arrived at my first assignment. Tried to take a break but my Commander wouldn't allow it. Lucky for me he pushed me. I graduated right before I rotated. Took three years to complete.