Almost immediately, so take care!
In theory, a hormone is released when you breastfeed that is similar to the type contained in the contraceptive pill. In practise, this may not be enough to inhibit ovulation and so a pregnancy can occur.
Those women more at risk of another pregnancy are the ones who have babies who sleep through the night (lucky them) so they are not feeding at regular intervals night and day.
Women who are not breastfeeding at all can very easily fall pregnant again.