The answer is that it depends on many factors, such as temperature and type of flour. Still, you can use these references and test the final result:
If you use a SHORT-FERMENTED BIGA (fermented for 16 hours), you can use it in a percentage ranging from 30% to 50% of the flour's weight.
If instead, you use a LONG-FERMENTED BIGA (fermented for 24 - 48 hours), then you can use a quantity of Biga around 20% - 25% of the weight of flour.