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For perspective, I finished my BS and AS degrees while enlisted in the Navy, took me 9 years from zero credits to completion. I didn't work that hard on it, honestly, if I pushed harder, I probably could have done it in 5-6 years comfortably ( as in, 2 classes a semester [each semester only 9 weeks long for my school, each class only meets once a week, with homework to do) However, during that time, I also worked very hard in my professional job and advanced from E-1 to E-6 in 7 years.
It's all about what your goals and motivation are at the end of the day. You want a balance of school and work success, or are you trying to go all-out school and get done within your first contract? (usually 4-5 years)
It depends largely on your field of work and field of study. Some jobs in the military follow daytime working hours similar to its civilian counterpart (e.g., medical, law, administration, etc.), while others include shift work and high deployment rates (e.g., security forces, transportation, civil engineering, etc). The location of assignment also effects your free time as some assignments have a high operations tempo which often requires longer and more flexible work hours from a strained work force, while other assignments experience slow operations tempo so having a dependable work schedule is more likely.
It isn't the branch as much as it is Active Duty or Reserves. The reserves from any of the branches should work best.