It varies from college to college.
The bond is of two types : bond against Seat and Service bond.
Once you are admitted to a medical college, you need to pay an amount of money to leave it.
Some bonds require you to serve the institution for a certain time or pay a fine
AFMC has army service bond for 7 years while ESI has it 3 years.
One needs to pay a certain amount if he/she doesn't want to work. 

• The Government has decided to prescribe a condition for admission to the MBBS Course of studies against the State Quota of MBBS Seats, that EVERY STUDENT WHO IS ADMITTED shall complete their MBBS Course from the Medical College to which he/she is admitted, i.e. you cannot shift colleges or course without paying the bond amount and

• that on the completion of the MBBS Course, he/she shall, if so required by the Government, serve in any part of the rural areas of the Country/State for a minimum period of 5 years on remuneration as may be prescribed by the Government.

• Now this bond period and bond money varies from college to college. In fact, some states do not compel to sign bonds in their medical colleges.

• I hope you find this helpful. All the very Best.