This time is going to be heavily affected by how you are running discourse. For example, if you are using vagrant across a shared filesystem there is a pretty good chance you are going to hit disk performance issues unless you’ve configured the share to use the correct amount of resources/watcher process to run the VM efficiently. Out of the box, discourse also doesn’t ship with a boot preloader (like spring) configured which will only reload the files you touch instead of the whole environment so you’ll be spending a fair amount of your time getting the environment loaded again.
If you’d like to explain a little more about how you’ve got it setup, I’d be happy to give you some pointers on speeding this up.