Old Mac and new Mac migrating 12 hours, how much longEr?

It depends on the connection. Apple points out in the support article on migrations that all you technically have to do to migrate between two Macs is to have them on the same Wi-Fi network and near each other if they’re running Sierra or higher. Try to connect them with an Ethernet cable instead (with the appropriate adapters if necessary). Or consider using a different type of cable and Target Disk Mode. If you don’t have the latest hardware and Wi-Fi router or your network isn’t running at optimum (which they usually aren’t), it could be slow, that's why there can be 12 hours of migrating and even more. I also had the same issue because I used Wi-Fi and can't work in Sound Studio for 8 hours. I was very unhappy because I bought new Yamaha electric guitar and wanted to work with its recordings...

How much data have you got? How are you connecting the two devices? Connecting over Wi-Fi can take a while (how long is a piece of string?) depending on the quantity of data. Quicker if using Ethernet.

Only you can really know how long it's going to take. Based on what you've given all we can do is give you a vague guess. A bit more information from you might help us narrow it down a bit. What exactly are you transferring? Photos? Music? Movies? Downloads? E-mail, Documents etc? How large are they? For example if you've a photos library 100s of GBs in size then its going to take hours and hours over Wi-Fi.