Sweeney tickets are always fairly easy to get.
I've rushed numerous times, and always gotten tickets. Basically, you could show up anytime before the show, but to be safe, I say get them by 6:30, an then go out for dinner.
"We don't value the lily less for not being made of flint and built to last. Life's bounty is in it's flow, later is too late. Where is the song when it's been sung, the dance when it's been danced? It's only we humans who want to own the future too."
- Tom Stoppard, Shipwreck