The SPY monthly chart shows two clear run ups and the resulting retracemet. The first cycle started in 1994 and ran up for 71 months and then retreated 76.4% in less than half the time it took to reach the high..
The second run up ran for 60 months and retraced 100% of the run up in 17 months.
Our current up cycle has been in place for 75 months and has had a nice run up. If we were to start a retracement period of the same proportion as the 1994 cycle, we would have 34 months of a down market and we would see the market retrace 76.4%.