How long does it take to transfer information from old Mac to a new Mac using the migration assistant?

An accurate answer is not possible since it depends on the aggregate amount of information to transfer, your local wireless environment (assuming you are doing it wirelessly), and other variables.

It's not unusual for it to take a full day or more.

We cannot tell you exactly. It is all about how much data there is to migrate… mine was finished in no time but I have not many Data outside iCloud at all.

how more data there is to migrate, the longer the process will take