How much tax I have to pay on returns (capital gains) for a particular IPO?

If any Capital Asset (stock, property, precious metal etc) is sold or transferred, the profits arising out of such sale are taxable as capital gains in the year in which the transfer takes place. 

Capital Assets are of two types i.e., long term and short term.

Shares, debentures and mutual funds are considered as Long-term capital assets when they are held for more than 12 months before they are sold or transferred. If they are sold or transferred before 12 months they are considered as short-term capital.

Different rates of tax apply for gains on transfer of the long term and short-term capital assets. Gains on short-term capital asset are taxed as regular income.

If an investor sells IPO allocated shares with in 12 month of IPO Allotment, he comes under short-term capital gains. All such gains are taxed along with the investor's regular income i.e tax on his salary etc.

For long term capital gains and more detail about taxation in India visit:

http://incometaxindia.gov.in/general/computation.asp