"How much should I invest/save for retirement?" is a pretty common question (try googling it). This question, like most other, is often asked with the expectation of a nice and comfortable answer. "Be sure to save 10-15% of what you make, for retirement" is an equally common answer. Unfortunately, nice and comforting as it may sound, such answers are often far from the truth.
Retirement planning is complicated because the corpus accumulated is not meant to be spent in one-shot like all other goals. The corpus must not only provide regular income to meet expenses, it must also grow at a rate that at least matches inflation and must be large enough to outlive the individual. This is a tough ask. Any retirement calculation is subject to several assumptions. If these assumptions are not realistic (for example planning with inflation of 6% and equity return of 15%) then results of the calculation would appear comforting even as reality prepares a bitter pill.