Here’s a table showing the most current metrics on the S&P 500. Also included are the historical mean and median values as well and the current values compared to them.

*All table values from multpl.com. Please see their website for associated tables and graphs.

Notes on the above table
1. Earnings, on average, have risen steadily since the 1870’s and are now nearing the all-time high of 95.32 hit in June, 2007.
2. P/E ratio has been widely fluctuating since the early 1990’s. P/E at all-time high of 123.79 in May, 2009. Historically, there’s been strong resistance at 22.5. For that to happen today, the S&P would have to hit 1983 (assuming the above earnings figure of 88.13).
3.Schiller P/E widely fluctuates. Though current price is high, it’s no where near 1999 all-time high of 44.2 and is still slightly below pre-recession high of around 27.5. (Data from 1880.)
4. Price/Sales nearing 10 year high. (Data from 2000.)
5. Price/Book hasn’t been above 3 since 2003. (Data from 2000.)
6. Book Value at all-time high (since 1999).
7. Dividend Yield falling on average since 1870’s.
8. Real dividend rising on average since 1870’s.
9. Dividend growth at all time high in 2012, just over 17.5%.

Conclusion
If we believe the Fed will continue its dovish stance (assuming Janet Yellen is sworn in as the next Fed chair) and barring any black swan events, we could see the S&P 500 continue to rally to the 1983 area. However, should the presumed scenario not play out, it’s not unreasonable to assume a sharp sell-off given the fact that many of the market metrics are well above their historical norms. How low could we go? Using the historical mean P/E of 15.50 and current earnings per share of 88.13, it’s very possible the S&P could drop 20% from today’s closing value of 1733 to the 1366 level.