You are doing nobody any favours with this question, you have not provided enough context for anyone to answer without guessing what the question really is.

Lets assume that is the time to reach maximum charge from flat for a charging model where the rate of charging is proportional to the difference between the maximum charge and the current charge.

Then starting from zero charge the charge state at time is:

Then we solve for from:

Then having found we solve for is:

(the arithmetic is slightly easier if we write the charging law as:

which you can try if you like)

RonL