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?

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.

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).

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.