Please tell me

• What is characteristic time in Fourier number?

• How can I calculate characteristic time?

Suppose I started heating water in a closed container by immersion rod. And temperature increases continuously, Suppose at time, $t=260 sec$ temp. at center is $40^oC$, and at $t=480 sec$, center temp is $42^oC$. Then to calculate Fourier number I need characteristic time.

• Then which time I should consider, $263$ or $480 sec$?