Yes, you can do that.
But you're not going to get 20 minutes out of it in one go, unless you clock it externally with a very low frequency of course. Instead you're going to set it up so it overflows 10, 25, 50, 100, whatever times per second and count the number of overflows, either using interrupts or by polling the interrupt flag - depending on how the rest of the program i structured.

/Henrik.