For what it's worth, this is the process I settled on.
Convert to a Retirement Fund Equivalent
The rule of thumb is that 4% withdraws are sustainable long term. So to get an additional 25k a year, your retirement fund needs to be: 25 / 0.04 = 625k larger.
And from there, to Years
Based on your target retirement nest egg, you can see how long it would take to ...