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.