Retirement can occur after 20 years of active duty service. Retirement pay is calculated based on time in service and rank only. The bonuses that active duty physicians receive are not included in retirement pay.

Service members can retire after 20 years of active service. Retirement automatically occurs after 30 years of service. The system is basically the same for all uniformed personnel. The base retirement pay is 50% the average of the highest 36 months of active duty base pay ("high-3") received. For each year past 20 years a service member receives an additional 2.5%. So for personnel who retire after 30 years of service they receive 75% of their "high-3".

There another option called "REDUX" where a service member receives a Career Status Bonus (CSB) bonus at 15 years (est. $30k) and then a reduced pension. It might be possible to make more money this way by investing the bonus.

The Office of the Secretary of Defense has useful calculators that can be used to look at the numbers:

http://www.defenselink.mil/militarypay/retirement/calc/index.html

Here is an example:
An O6 brings home about $8000 / month before taxes. This equates to an annual salary of $96k. A rough estimate for the monthly retirement pension would be $96k * 0.5 / 12 (Roughly $4k per month).