How long should it take to migrate from one Mac to another?

If both Macs are running macOS Sierra or later, you can bring them in range of each other and they will set up an Ad-Hoc network to transfer your files reasonably quickly. that could still take all afternoon.

if you have other conditions, connecting over Wi-Fi can take several days.

Connecting over Ethernet through your Router can be faster.

Using the old Mac's Backup drive, and connecting it to the new Mac is fastest of all.

One further faster option, before having to use a backup, is usb-c to usb-c to connect the two computers.

USB is a local peripheral interface. USB does not work for computer-to-computer transfers.

Some users report this worked for them, but was very slow. That is because it was not actually working as expected.

If the interface is not listed in System preferences > network, then you are NOT actually using USB for the computer-to-computer transfer. Instead, you are using the TOPMOST, Working interface shown in system preferences > networks

Wow, thanks Grant.

It did take longer than I would have expected.