The time varies depending on a lot of factors. These can range from the capabilities of your computer to the current length of the chain. For this reason, it's hard to give a blanket answer as it always varies.
But there is a silver lining, you can drastically reduce the build time by using the binary cache setup by iohk: https://github.com/input-output-hk/cardano-node/blob/master/doc/getting-started/building-the-node-using-nix.md#iohk-binary-cache
Hope this was helpful.
But you can figure it out.
Step 1) using CardanoScan to figure out the current epoch and the current slot it will also display the number of slots in an epoch.
Step 2) let the build process continue for a while a time how long it takes to reach X slot. The larger the X you chose the better the approximation. The amount of time is called t.
Step 3) ((current epoch x slots in an epoch ) + Current Slot) * t/X approx How long it will take.