Unlike others who are making fun of this question, I will try to answer it genuinely. You have a target amount and that’s great. You have a rough target time as well. Alright! So when people say that they wish to invest per month over a long period of time, I usually suggest going with Mutual Funds. That is up to you to do your own research on what Mutual Funds will be good for you based on your risk appetite and how well you understand that sector; roughly a good and diversified Mutual Fund portfolio gives you a return of around 12% per year.
So if you expect a return of 100 crore rupees at the end of 15 years, at an average return of 12% per year, you need to invest around 20 lakh rupees. If you extend your target to 20 years, you would be required to invest 10 lakh rupees every month.