Vinay,
There are many ways of doing it. The most popular are: 1) basic salary-based (Christmas and mid-year bonuses) and 2) percentage of basic salary (productivity-based). Salary-based bonuses range from half a month BS to 3 months BS. Percentage-based bonuses range from a low of 0% to 300% depending on the program and budget.
In our part of the world, bonuses are annually given by employers towards mid-year and at the end of the year (usually Christmas and from 0.5 BS to 3.0 BS). This is a way of sharing a company's profits with its employees.
In Japan, they also have it salary-based and given twice (Summer & Winter, both 3-months BS). US companies prefer productivity and performance-based bonuses, although given also at the same time of the year (midyear or Christmas time).
Best wishes.
Regards, Ed Llarena, Jr. Managing Partner Emilla Consulting