How long does it usually takes to transfer all files from an existing Mac to a new Mac?

Yes, it depends on the folders that are currently being transferred, for example a folder with 1 GB and thousands of small files takes much longer to copy than a 1 GB movie file. So while it is copying folders with lots of small files the estimation will go up (and it will come back down when bigger files are copied).

Depends on method used &how many GB of data, Wifi is slowest, ethernet faster, USB3 drives faster, TB connection even faster.

That would depend on how you are transferring the data and how much there is to transfer.

Yes, that is normal as it looks for files to transfer, goes back to transferring then looking for mre that need transferring.