How long are the service obligations for prior service soldiers going back into the Army?

If you have already met your 8 year mandatory service obligation you do not have to worry about that again. The 8 year MSO is only for the first time you sign up. Your Contractual Obligation on Active Duty depends on what is offered at MEPS at the time you go in. Best things is talk with your local recruiter and see what they can offer you.

I just switched from Marines to army and they gave me 3 years 16weeks for some odd reason.

I was in the Marine Corps and I just joined the Army it was a quick process.