This depends on a number of factors. For the most common domains, here's how it works:
Once your existing provider has made the change, the domain will normally arrive with us within a few hours at most. We will confirm with you via email when this has happened.
Once we initiate the process at our end, it can take up to 6 days. This can normally be improved by approving the transfer on your existing provider's control panel. Please note that we can't help or advise on the specifics on how to do this!