Retirement planning is invariably part of any investment strategy for individuals aiming to maintain a steady flow of income. Among many instruments available nowadays, the National Pension Scheme (NPS) has emerged as one of the best investment options to build a retirement corpus.The key question most investors face is: How much do I invest to earn a particular amount of monthly pension? What will be the current outgo per month to build the corpus and time horizon for that investment?For starters, the NPS, as we know, is the social security initiative of the central government. An NPS account holder can claim income tax exemption on up to Rs 2 lakh investment in a single financial year — up to Rs 1.5 lakh under Section 80C and an additional Rs 50,000 under Section 80 CCD.Once the amount matures, the investor has to decide how he/she like to invest in an annuity. The annuity amount decides the amount of pension an investor would receive, said Abhinav Angirish, Founder, Invsetonline.in while speaking to CNBC-TV18.For example, someone looking to earn a pension of Rs 1 lakh per month. CNBC-TV18 discussed with experts to find the answer. There are several factors at play as well.“For one, NPS allows the investor to choose investment options. For example, the investor can choose 60 percent equity and 40 percent debt. These investments will be invested in the default funds handled by the Pension Fund Regulatory and Development Authority. This balanced mix of investments can yield around 10-10.40 percent return. Assuming the investor has invested for 30 years, provided he/she invests 60 percent of maturity amount in an annuity, and assuming the annuity return of 6 percent the investor needs to invest around Rs 15,000 per month to get a pension of Rs 1 lakh,” Angirish stated.ALSO READ | NPS vs SIP: Which is a better investment plan?In short, it is actually the time for which one is making investments matters and the rate of interest.For instance, if an investor has 20 years at hand to build this corpus, then, at 12 percent returns, he/she needs to invest Rs 32,000 each month for 20 years to build this corpus, said Adhil Shetty, CEO at Bankbazaar.“But if the investor has only 10 years, he/she would have to invest a whopping Rs 1.3 lakh every month to build the same corpus. At 15 percent, the investor would be able to build the same corpus in 18 years if he/she invested Rs 32,000 each month," he told CNBC-TV18.A stark reminder for investors is to factor in inflation as well, Shetty cautioned.At 6 percent inflation on average, he said that the value of Rs 1 lakh after 45 years would be equal to approx Rs 7,500 today. So, despite getting Rs 1 lakh at the age of 85, it may not be sufficient at all to meet the requirements.There still is a way out to negate the impact of inflation, said Investoronline's Angirish adding that if an individual invests just Rs 10,000 per month in equity funds, he/she can amass the corpus of Rs 6.92 crore over thirty years.This will enable him/her to invest 100 percent of the maturity amount towards annuity thereby getting a higher pension while taking care of emergencies and other obligations, he said.Disclaimer: The views and investment tips expressed by investment experts on CNBCTV18.com are their own and not that of the website or its management. CNBCTV18.com advises users to check with certified experts before taking any investment decisions.