Depending what country you live in the amount you require in retirement will vary greatly however the principle is the same. The Association of Superannuation Funds of Australia states that a single retired person living a ‘modest” lifestyle needs $22,000 pa while a couple needs $31,500 pa (rounded). Choosing a “comfortable” lifestyle means singles need $40K and couples $50K. You can see what is meant by these figures and how they’re calculated at http://www.superannuation.asn.au/resources/retirement-standard
My recommendation is record your own expenses (including a weekly amount for annual expenses) to see your total weekly expenses amount. After deleting those expenses you won’t have in retirement (eg the mortgage, provided it’s paid off), you’ll have your weekly “retirement requirement” after tax amount. You can then go to an online calculator to work out how much of a nest egg (lump sum) you need in order to sustain your lifestyle for the number of years you expect to remain on this plane(t).