\(U=U_0(1-e^{\frac{-t}{RC}})\\ \frac{U}{U_0}=1-e^{\frac{-t}{RC}}\\ e^{\frac{-t}{RC}}=1-\frac{U}{U_0}\\ \frac{-t}{RC}=log_e(1-\frac{U}{U_0})\\ -t=RC*log_e(1-\frac{U}{U_0})\\ t=-RC*log_e(1-\frac{U}{U_0})\\\)