Employment bond is a contract made between the employer and employee .In addition to the terms and conditions of employment it also contains special clause for conditions when the employer either offers special training to enhance the employee's competency and utility to the company or defrays special expenses in deputation abroad etc. All this requires substantial amount of investment.

Since the employer has made an investment in the employee he feels and probably is justified to get appropriate contributions (value addition) from the employee in return. Hence the bonds are drafted to bind the employee serve to the company for a defined period (within which the employer feel the money invested would have been recovered adequately).

There cannot be any mathematical formula or generalized rule to decide the period for which the employee may not leave employment and the value of the bond he may be required to sign. It should be reasonable and be seen by the courts as such if there be any litigation in future. Experience tells that it is uneconomical to enforce the bond; it is time consuming
and at best a psychological deterrent only.

That might explain why companies sometimes do not follow up with the employees who break bonds and move out especially when the employee is moving for higher studies. However when the employee is of significant importance to the firm employer might choose to enforce the bond. However in that case the bond needs to be proved in the court as a balanced one and which is reasonable in scope. Since all employees had freedom of mobility the bonds might not be enforceable. Employers need to have offer better motivational tools to retain employees than attempt to do so through bonds.