Brute force method to solve the 0-1 knapsack problem
You can't. Landau notation does not hold enough information.


You don't know the constant factor(s).
With only $O$, you don't have a lower bound. 
It's only a worst-case bound; different runs with the number of items can have wildly different runtimes. 
Such bounds only hold in the limit, that is even $\leq c \cdot 2^n$ may not hold for those $n$ you can realistically run the algorithm for.
The quantity bounded is not runtime but some abstract measure.

In particular, effects like operation reordering, memory hierarchy and branch mispredictions routinely skrew with runtimes.


The best you can do is assume that the runtime behaves like $\approx c \cdot 2^n$, run experiments with $n_1$ and $n_2$ items and estimate the constant factor -- then it's simple rule of three. But given all the caveats listed above, the prediction is unlikely to be worth much.