The easy way to do this is with an excel spreadsheet.
Put 24,660 in the first row. and put 24,660*(1.01) - 414 in the row below, and spread it down until the number goes negative.
The other way to do it is to derive a formula.
The NPV of future cashflows is:
$414 \sum_\limits{i=1}^n (1.01)^{-i} = 24,660\\\ 414 \frac {1-(1.01)^{-n}}{0.01} = 24,660\\\ n = -\frac {\log(1-\frac {24,660}{414}0.01)}{\log 1.01}$