Let us assume bank follows the process of Simple Interest.
Simple Interest = Principal x Interest Rate x Number of years of deposit / 100
Let us assume Principal amount is Rs.1000 To make it double, interest earned has to be Rs.1000
By using above formula, Number of years of deposit = (100 x 1000) / (1000 x 8) = 12.5 years. Using compound interest.
Most banks calculate interests by compounding them either monthly/ quarterly/half-yearly or annually.
When amount doubles, to calculate the number of years of deposit, we may use 70/interest rate(r) rule.
70/8 = 8.75% approximately.