Automatically going for 'most commits ever' as well, I guess:
https://github.com/smaslennikov/whattimeisitrightmeow/commit...
After that, if you assume 128 bytes per commit (timestamp, and maybe a hash or two saved?) then probably around 14 years.
I modified the script to commit as fast as possible (cycling the minutes between 1 and 60), and after 600 commits, doing "git gc --aggressive" before and after, the .git directory grows by around 460 bytes per commit.