Interesting question. My best guess is not for a long time. We're still at a stage where there are enough people asking this question. Once nobody is asking what the downside is - that's when we'll hit levels like 3600 or lower.
My caveat is this: the interest rate cycle will turn and when it turns - that will be a much truer test of market strength than anything we've seen to date. I suspect we will see a 12-14% correction in a week, as soon as tightening becomes apparent. This is a market running on excess liquidity. Yields one year from today will hit 2007 levels and once that happens, equities will be an awful place to be stuck in. I'm not a long-term bull, but I do intend to enjoy the bull-run while it lasts.