Over the last 20 plus years the average Bear markets run 7 months, the reason it is that high, is because of the Big Bear markets of 2000 & 2008, the average Bear, establishes a low within 5 to 6 months, but in the last 20 + years, most of the Bear market lows have been established within 1 to 2 months.
Also note that the average Bull market over the last 20 + years has averaged 21 months, the SPX is currently in the 19 month of this Bull market.
SPX Major Market Trend Periods (SPX Monthly EMA 3/8):
If you look at the SPX Monthly Cycle Data Sheet below, you will see that the average Monthly Phase 1 over the last 20+ years have an average of 6 months, it's possible to have a Monthly Phase 1 without a Bear market confirmation. The average low number of months is 4. If you notice the Bull market of 2003 had two Monthly Phase 1's each lasting one month and declining 8% to 9%.
SPX Monthly Cycles:
The SPX current Bull market still has two more months before it extends, the SPX Monthly Phase 2 is currently extended by 4 months, the SPX Weekly Phase 2 is now in it's 3rd week (average 11 weeks), the SPX Daily Phase 2 is now in it's 7th trading day (average 9 trading days), and the SPX 60 min Phase 2 (average 9 trading hours) has currently completed 44 trading hours, which is now the current record and looks like will continue today.
Bottom line: So the SPX is way over due for a Phase 1, the Daily Phase 2 will likely extend, the Weekly Phase 2 will likely at least be an average one which should take the SPX to 1708 by end of August, so the earliest for a Monthly Phase 1 right now is September/October and a possible drop of 17.66% which is the current average.
You said, "The odds favor a 20 percent drop followed by another big rally" I agree. I believe this Bull market could last for another couple years or more. Like the Bull markets if 1995 & 2003.
Market Data 
Markets 
AI
