The hydrolysis of sucrose was studied with the help of a polarimeter and the following data were collected:

| Time (min.) | $0$ | $70$ | $\infty$ |
|---|---|---|---|
| Observed rotation (degrees) | $44$ | $16.5$ | $-11$ |

The time taken when the reaction mixture will be optically inactive? (Given: $\ln 2 = 0.7$, $\ln 3 = 1.1$, $\ln 5 = 1.6$)