The answer to this question is different for each person.
I think it is a 3 step process.
1. You need to think about what your dreams are for your retirement. Do you want to travel? Do you want a second home in FL? Do you want to stay home and take up a new hobby? Are you planning to live simply? et'c. et'c. The purpose of this step is for you to figure out how much money your are going to need.
I attended a retirement planning workshop and one of the planners said one way to estimate what you will need in retirement is to take what your taking home now and subtract what you are saving--the resultant figure will be what you are spending right now and the figure you should target (keep in mind that I am only 5-10 yrs from retirement and my current income is probably pretty close to what I will be making when I retire--you have a few years to go and your income will most likely be significantly higher in 20+ years from now.) Of course, if you have an ambitious retirement planned, you might have to save more. On the other hand, there may be other expenses which you will not carry into retirement such as Union Dues. Will your house be paid off? If so, that will be one less expense you will have to pay. Will your kids be on their own by then?
2. Figure out how much money you have already saved. Will you have a pension of any kind? If you receive a pension, then you will only have to make up the difference between what the pension pays and what you will need to derive from your savings. Anyway, this is where you figure out what you will need to save to replace your income.
3. The next step is save money for your retirement goal. If you don't meet it 100%, you are still closer than you were before. If you take a snapshot and you aren't meeting your goals, it might mean delaying retirement a little bit.
You are already taking the first step--trying to figure out what you will need.
-DC