How long will a corpus last depend on two factors – the amount of regular withdrawal and the rate of return on the invested money. If your withdrawal is lower than the rate of interest earned on the invested money, your corpus can last forever. For example, if you can invest Rs 1 crore at 6% p.a. after tax, you can withdraw up to Rs. 6 lakhs without depleting your initial corpus. However, if you withdraw Rs 7 lakhs p.a. your corpus will last about 28 years.