Financial planners often use that argument ("return of x% per year").
However, real-life is more complicated: the market may be up 10% one year, then down -15% the next, then up 2%, then back down -3%. You get the idea... So assuming "x% yearly over y years" is unrealistic. Depending on how the sequence of returns line up in time, you will end up with vastly different numbers in the end.
The "x% yearly over y years" model is deterministic. A better way to forecast is to use a stochastic model or a Monte-Carlo simulation. There are a few free ones floating around. You could also write your own in Excel but it takes time, depending on how detailed you want it to be.
With everything said, it's still a good idea to put away as much money and as early as you can, while not denying yourself some pleasures of life.
(Disclosure: I have a MBA with a concentration in Finance)