App Store: Not enough disk space to install Xcode
There's really no easy way to produce a definite answer for you. The definite answer varies from system to system. 

You could get the definitive answer by freeing a small amount of disk space. Taking a full backup of your system. Try installing. When it fails, restore from backup. Repeat until you have the definitive answer.

It would be an unrealistic amount of work to get to a number that you would then have no interest in having, as you have already installed the software.

EDIT: You have changed your question so that you no longer require a "definitive" answer, but rather want an "approximate answer". To that I can I add that my installations of Xcode are approx. 15-20 GB in size depending on what exactly is installed on each computer. Note that for updating you'll need room for both the currently installed version plus update files. You might find that it is easier on your disk space requirements to simply delete Xcode, and then reinstall it. You might get be with even less disk space if you download Xcode manually from the Apple web site instead of from the App Store.