well you probably want at least 5gb. Updates and stuff install onto the same partition. And you should always have some free space on the OS partition.
But 5gb should be enough.
What I usually do is maybe a 100gb or so for the OS and then 200gb for storage. Then I install all my programs on the OS partition, since you will have to reinstall them anyway if you screw up. (well you could just mess with the registry, but I think it's easier to just reinstall) The storage will have all the things I don't want to lose if I have to reformat.
But that's just me, as we already said something like 5gb should be enough for the OS.