Sum of Geometric series from a and r

How does one compute the no. of terms $n$ in a geometric series given the first term $a$, the ratio $r$, the sum of the series $s$.

The answer found corresponds to $n = \dfrac{\log\left(1+\frac{s(1-r)}{a}\right)}{\log r}\quad$ (for $r<1$)

Does this solution have any limitation in finding n ? or what is the error in finding actual $n$ and $n$ computed from the solution ?