Nifty PE ratio measures the average PE ratio of the Nifty 50 companies covered by the Nifty Index. PE ratio is also known as "price multiple" or "earnings multiple". If P/E is 15, it means Nifty is 15 times its earnings. Nifty is considered to be in oversold range when Nifty PE value is below 14 and it's considered to be in overvalued range when Nifty PE is near or above 22. The market quickly bounces back from the oversold region because intelligent investors start buying stocks looking to snatch up bargains and they do the exact opposite when Nifty P/E is in the overbought region.
Check out what Professor Bakshi (a famous Indian value investor ) has to say about Nifty P/E. Recent research done by my firm shows just how dangerous it is to remain invested in an expensive market. Since NSE started, every time when Nifty's Price/Earnings ratio exceeded 22, the average return from Indian equities over the subsequent three years became negative.
Source - sanjaybakshi.net
History clearly tells us that if you are a passive long term investor you should buy stocks when P/E reaches 15-16 and stop buying when P/E goes above 22. If you are not comfortable buying individual stocks then you should buy Nifty Bees ETF. Nifty Bees is like a mutual fund which tracks the NSE Nifty Index.