How long does it take 1 TB to transfer from an old iMac to a new iMac?

You can reduce the time and get far greater accuracy by using a wired USB connection to do your transfer. Wi-Fi will often cause corruption and it is far slower. A transfer should not take all day. Very likely it will complete in an hour, more or less.

Too many variables with too many unknowns. No one can tell you how long it will take, and even if that were possible, Migration Assistant will do whatever it can to spoil that estimate. When it is done.

I've been trying to transfer about 600gb via wifi from Thursday afternoon to Monday morning. It's still not done. I'm at about 85% complete at this point.

I wish I had bought a cable, but I'm running from an older model Time Capsule and without being able to go into a store to see the connection myself, I wasn't chancing wasting money and having to return it.

Super frustrating to have a brand new computer that's unable to be used yet.