This a tough question to answer and if anyone gives a pat response then do suspect them! Even Warren Buffet famously said that he can’t predict the market.
However one can say that good equity funds have given about 15% returns over the last ten years and given the huge potential in India ahead, such a number might be sustainable. From a planning perspective, please assume 12% to 15% mutual funds.
Invest in direct plans with professional advice (check Jama.co.in). Else over these 15–20 years you end up losing 40% of your investment to the brokers.