How to estimate maximum n given complexity and amount of time?
You can't.  Mathematically, you can't conclude anything about the running time for all $n$, given just two points (given the running time at just two values of $n$).  There are many possible curves that might go through those two curves; two points aren't enough to determine a curve.  The running time might be something like

$$\frac{1}{2^{100}} 2^n + n^2.$$

Or, it might be something like

$$\frac{1}{2^{95}} 2^n + \frac{1}{10000} n.$$

Or, the running time might be

$$\frac{1}{100000000} n^2.$$

Or any number of other possibilities.



That said, in practice it seems very unlikely that you have an algorithm whose running time on all instances of size $n$ is $\Theta(2^n)$... and yet for $n=100,000$ it completes in 16 seconds.  That's a bit silly.  In practice, it could happen if the algorithm is sufficiently weird, but not bloody likely in practice.