migrating from hard drive to iMac

Until it's done, however assuming you are using Migration Assistant to migrate either a Time Machine External Hard Disk or a bootable clone External Hard Disk and the External Hard Disk is connected to the new Mac via a USB cable about 2 hours is a safe guesstimate. .

A while.

All depends on how fast both drives are , and what connection you have to the external hard drive.

Glad it worked okay. It always depends on so many variables that getting an accurate estimate is difficult. Variables include the amount of data and the speed of the transfer.