If you've got the time, start working on it now. There are always, always revisions and improvements to be made and it'll definitely be to your advantage to have it completed so you can apply as early as possible (which is absolutely crucial). The PS was the main factor holding back submission of my app this cycle, leading to late secondaries, late interviews, and a waitlist at my #1 school. While I have a decent rank on the list and will probably get in, I would have much preferred to hear the good news back in November rather than May.
Anyway, that's my pair of copper Lincolns...