@pattu’s study concludes that if one invests at a NIFTY PE level of 25 and above, it would take about 5 years to get a small but positive return. Other than that, there wasn’t a strong correlation between NIFTY levels and returns. In fact, there have been only 3 occasions (2000, 2007-08, 2010) that NIFTY PE has breached 25 between 1999 and 2014…so the study still isn’t entirely conclusive. That’s a very small sample. Still, it makes sense (to me) to avoid buying when Nifty PE is very high.
If buying only when Nifty goes below average, booking profits when Nifty is high is your game (and you stick to it), more power to you. Wishing you all the best!