lol right! Seriously what decides it all is the choice of how you want to live. Like do you want to live w/ the same style after retirement as before (after you figure your reduced costs & your reduced income) or is it ok to accept a cut in lifestyle. If you decide you want no change, then you figure what cut from your present income will give you the savings you need to generate the retirement you want to live on. Return on investments at 8% is believable, and while I've been doing better than 8% I'm honestly not sure how much of that's dumb luck.
At any rate, if you know how to do simple math on a excel or some other spreadsheet (many are free) then you make a column for years, another for income and the 3rd column for the balance and it tells you what you need to save. Let me know what you find, it's one of my favorite pastimes.
I would add an expense column (which would be a sum of a detailed expense report, i.e. insurance (home, car, umbrella), college, property tax, food, clothes, vet, medical, utility, etc. costs) with an annual increase of 3%. Also, for the sake of being conservative I would use a ROI (return on investment) of 5%. That way if you get 8%, YIPPEE you are ahead of schedule and can retire earlier. : )