Can anyone tell me how long it took to transfer from old to new iMac using WiFi connection?

Please NEVER EVER use Wi-Fi to migrate, it will almost certainly fail. If you have started and it has been going over 2 hours, abort it and please follow the directions below:

Connect the old Mac's Time Machine External Hard Disk using a USB cable to the new Mac. If you do not have a Time Machine back up for the old Mac, create one ASAP. Once the Time Machine External Hard Disk is connected to the new Mac using a USB cable launch Migration Assistant and follow the on-screen prompts. Why do it this way, simple it works every time and is the fastest method there is. If you do not know how to launch MA it is located in Applications - Utilities - Migration Assistant. You can also launch it using Spotlight, click the Spotlight icon in the upper right of the display and type Migration Assistant.

The overall time to transfer data between Macs can vary greatly and is based on a number of factors, including the overall bandwidth of the connection between them and how much data is being moved. It can range anywhere from a few to a number of hours. There really isn't any average to speak of.

Are you having an issue with the migration?