I have been retired for four years. I did as you say. I started with historical spending patterns as a base for my retirement spending projections. Then made assumptions about investment returns, inflation and taxes. I am an accountant, so the complex calculations were manageable. If you are not an accountant and not adept with spreadsheets, I would recommend hiring and independent financial advisor. I hired one, just to double check my work. It a takes about an hour a month to update my expenses and redo my projections. This must be done every month. I manage my own investments too. As they are primarily equity investments, monitoring them takes about an hour a day. If you have someone else monitor your investments and or invest in broad based passively managed, index mutual funds or ETF's then less time monitoring your investments will be required. Good luck.