How long does birth control usually take in order for it to be effective and prevent pregnancy?

Depends on the method of birth control

If you're referring to the contraceptive pill, here in the UK we're advised to take it for at least 7 days before having sex to prevent pregnancy. Though there's always still that percentage anyway.

X Dani

As far as the birth control pill, we recommend 30 days for maximum effectiveness. However, remember that there is no form of birth control that is 100% effective.

Yes, at least here in the US, the recommended time is 30 days after you start taking it. And even taking it, there is still a risk. No method is 100 percent

Right, in that case, it's best to check with your doctor as you're prescribed the medication, what the recommended length of time is.