The longest I've had to wait was 2 hours.

If you want to make SURE it doesn't interrupt your playtime, initiate the transfer after you log off at night, and then wake up in the morning to a completed xfer.

It helps to keep the character transfer status page up and refresh it. Sometimes the confirmation e-mail can take a while to reach it's destination -- but as soon as you finalize the transfer on the character transfer status page you should be fine.