Pilot is right but to add on.-
You only accrue G.I bill days when you are on full active duty, everything else is considered tuition assistance. The amount you server on active duty does correspond to the amount of schooling you get paid.
To answer your question about how much they will pay, they pay up to the largest public university/college in-state tuition rate. Generally that does not cover the cost of medical school.
Also with that, if your graduate school is attached or in the same program as an undergraduate school i.e University of Colorado Denver provides undergraduate and graduate programs, the G.I Bill will only pay for the undergrad amount but you can apply it to your medical school instead.
(I understand it's a lot and it's messy and I may or may not have a tendency for run on sentences)
To clear anything up, from the VA website-
"If you have at least 90 days of aggregate active duty service after Sept. 10, 2001, and are still on active duty, or if you are an honorably discharged Veteran or were discharged with a service-connected disability after 30 days, you may be eligible for this VA-administered program."
Furthermore here is a tool from the website to calculate (you can also include what school your interested in and it will show you how much of it they will pay) as well as a pamphlet with more information on it.
Tool (Select Veteran as that is what you will be after your contract) -
https://www.vets.gov/gi-bill-comparison-tool
Pamphlet -
http://www.benefits.va.gov/gibill/docs/pamphlets/ch33_pamphlet.pdf
