It is impossible to perform $100\%$ of the reaction, whole of the substance never react. Hence, time taken for $100\%$completion of a reaction is infinite. 

$$\begin{gathered} \mathrm{t}=\frac{1}{\mathrm{k}}\ln \left(\frac{{\mathrm{N}}_0}{({\mathrm{N}}_0-\mathrm{x})}\right), \\ {\mathrm{N}}_0-\mathrm{x}=0, \\ \mathrm{t}=\frac{1}{\mathrm{k}}\times \mathrm{Infinity} \end{gathered}$$

So, time taken is infinite to complete the reaction.