I usually leave about 10% free space, but some say up to 20% is best.
Make sure to optimize your drive by turning off hibernate, disabling system restore and if you have at least 8gb of ram (and don't run VMs or any real memory intensive apps) either turn off, which I don't recommend, or as I do recommend move your paging file to a HDD as it will rarely be used and just wastes space.
And take a look at this: http://thessdreview.com/ssd-guides/optimization-guides/the-ssd-optimization-guide-2/