It is taxed in a first in first out (FIFO) method. This means that if you redeem part of your SIP investments, the earliest instalments are redeemed first and the later instalments are redeemed later. For instance, if you made 36 monthly instalments and you want to redeem part of the investments, your initial instalments (first, second, third and so on) are redeemed first.
This means that in order to qualify for long-term capital gains, each of your instalments in the invested corpus must complete at least one year (in case of equity or equity oriented schemes). For equity or equity oriented schemes, long-term gains (those made over one year) over Rs 1 lakh in a financial year is taxed at 10 per cent and short-term gains are taxed at 15 per cent.