The phrase "difference in length" refers to the measure of how much one thing is longer or shorter than another thing. It shows the contrasting distance between two objects or measurements. Full definition
Since you are withdrawing, a significant drop early in retirement makes a BIG difference in the length of time your nest egg will last. (allfinancialmatters.com)
There are also differences in the length of time it takes for children to move from stage to stage. (cyh.com)
They're the same except for the obvious difference in length. (kingofthegym.com)