It depends on where the domain is currently held, and where you want to transfer it. Typically once you have paid, and we know where you’d want your newly purchased domain name to be transferred to, it should not take more than 2 days.
The fastest way to transfer is to use the same domain registrar as the current owner. If you choose to transfer to a different registrar from where the domain is currently held, you might need to wait an additional 5 days to cover ICANN’s mandated waiting period.