A sum of money doubles itself in $4$ years at compound interest. In how many years will the same sum become $8$ times at the same rate of interest?

Under compound interest, if a principal amount becomes $2^1$ times in $T$ years, it will become $2^n$ times in $n\times T$ years.
Given:

* The sum becomes $2$ times ($2^1$ times) in $T=4$ years.
* We need to find the time required for the sum to become $8$ times.

Expressing $8$ as a power of $2$:

$$8=2^3⟹n=3$$

Therefore, the total time required is:

$$\text{Total Time}=n\times T=3\times 4=12\text{ years}$$

Thus, the sum will become $8$ times in $12$ years.