Have you heard of the rule of 72? Basically, rate of return x time in years to double = 72. So if you get a 7.2% return, your money will double in 10 years; if you get a 10% return, it will double in 7.2 years. Etc.
There are two things to consider. First, if you are looking at something like "time to $X," the first $X takes forever, and the ensuing ones get faster and faster, because each $X becomes a smaller and smaller portion of your overall investments. So for ex., say you have $100K and you get a 7% return, so it doubles in 10 years. Well now, at that same rate, it would double again in another 10 years. But at that point, you're doubling $200K into $400K -- NOT just adding another $100K on. So it will take your money far, far less time to generate $100K than it did the first time, because you're working from a larger base. However, the time to DOUBLE the current amount in the account stays the same, as long as the interest rate you receive is the same.
The second thing to consider is probably what is happening here: I am assuming you're not just letting some existing money ride, but instead are adding to it over time. So maybe that first $100K took you 10 years. If you let it sit all on its own and get a 7% return, per the above, that money would double on its own in the next 10 years. But you're not letting it sit -- you're still making additional contributions as well, and those are also growing. So your $100K will become $200K much sooner than 10 years from now, because you're adding more money to help that growth along.
Make sense? If you want anything more detailed, there are any number of calculators on the web that can tell you how long your money would take to grow from X to Y if you continue to contribute $Z each month.
