A good rule of thumb for me that it's time to put a script down during an intensive rewrite is when you can't see the wood for the trees. In other words, those other words could be other words and you wouldn't even know it!
After doing a new draft for various submissions I literally can't "see" the script anymore. I've been over it so many times - tweaked and changed and refined and polished - that I need to step back and let it sink in. It's simply time to stop. Which is why having other projects to work on is invaluable so there's no downtime when you're burnt out on one script.
The draft is pretty much locked unless there are any last minute 'saves' spotted by my creative colleagues. Then it's off to submission land which leads to waiting interminably for news land which is kind of like sitting at a bus stop at 3am - one's gotta be coming soon or later ...
While that's trundling along the freeway, I'll resurrect an old script and give it a spit and polish. Not to mention the development workshop next week which requires some pre-work, namely watching Remains of the Day and Little Miss Sunshine, two movies I haven't seen.