It is an option at the beginning of a new campaign in version 1.5 of RTW, and I think (not certain) in version 1.3.

If you are running Rome Total Realism, then when in RTW you might be in version 1.2 of RTW, which I do not think has the option to turn off the battle timer.

I could be incorrect about that, but I am sure someone here will correct me if that's the case.