I am cloning the Linux Git repo from GitHub as we speak:
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Receiving objects: 34% (3615341/10423780), 3.66 GiB | 5.30 MiB/s
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That's only the master branch that has 1.3 million commits. Looks like it will be about 10.5 GB total!
Edit: It only came out to about 5 GB and there was an error on the last checkout step because of an invalid file name on Windows.