Minimum 7 days read the leaflet of the birth control and it will tell you.

However most birth control is only 98-99% effective when taken 100% correctly.

This means out of every 100 girls using the birth control completely right 1 or 2 will still get pregnant.

So the best option is to use birth control and a condom if you want to avoid pregnancy.

Remember birth control doesn't prevent spread of diseases.