Not to jack the thread, but it's compounding interest which can a problem, a good problem, but nonetheless a problem. So if you're making 15% the money will double in 4.8 years. If you're making 6% the money will double in 12 years.
So someone who decides to retire at 59 1/2 has 12.5 years until they hit the age of RMD at 72. So the portfolio will more than double for most people before they hit the age of RMD. Of course funds are likely to be withdrawn so the portfolio will decrease.
To be in the 12% tax bracket, a married couple will need to stay below $80k. Let's say they both are drawing SS at 20k each. That leaves 40k they can withdraw or convert and stay in a low federal tax bracket. If one or both get a pension, let's say 20k, then only 20k can be withdrawn per year to stay in the 12% tax bracket. (I'm of course ignoring any deductions for this oversimplified example.)
Let's run another example. A husband and wife have been heavily contributing to 401k and are in their 50s eyeballing retirement. Most of their money is in traditional 401k, not Roth. Let's make them the same age, 50 with a million combined in retirement. That's 22 years until RMD. If they are averaging 10% per year, then in 22 years 1 million will become 8 million. (It will double 3X in 22 years at 10%) The RMD at 72 is roughly 401kfunds/27. That means if they haven't done anything at age 72 they will be required to withdraw $300k which is the 24% tax bracket and not the 12% they anticipated.
Here's a real world example of compounding interest. I know someone who left $2,500 with an employer's 401k in 1998 because the amount was small and their new employer didn't have a 401k at the time. They never put another dime in it, but they did/do monitor the investments and made changes over time. Today, 23 years later, that account has $85k in it.