Just got a new iMac and I’m trying to transfer information from my Mac mini using Migration Assistant. How long does this process normally take?

For my M1 Mini data transfer from Catalina 2012 Mini - I tried Migration assistant over Wifi and it took forever and stopped.

I ended up using a Thunderbolt cable w/ Apple TB3-TB2 adapter and transfer done in 20 minutes (175GB).

Using Wi-Fi over your local network is slow and unreliable. If you have a lot of data to migrate, you will be lucking if it ever completes at all.

Ideally the best way to migrate to a new Mac, is to connect the old Mac's Time Machine backup to the new Mac during the initial setup.

To answer your question, if you are transferring between computers using a USB cable, I would estimate an hour, more or less, depending on the volume of data to be transferred.