Calculator Graphing Window Comparison

I have been asked to see for what range of $\displaystyle x$ values $\displaystyle p(x)=1+x+(x^2/2)+(x^3/6)+(x^4/24)+(x^5/120)$ provides a good approximation of $\displaystyle f(x)=e^x$, when I graph them with a window of $\displaystyle -10\leq x\leq 10$ and $\displaystyle 0\leq y\leq 200$, the two lines look fairly close at the top, but I know that $\displaystyle p(5)=91.4167$ and $\displaystyle f(5)=148.4132$.

I know that the lower limit of my answer is 0, but I can't figure out quite to find the upper limit. When I use the graph, do I look at differences in y values for specific x values, or differences in x values for specific y values?