Strahd defeated outside Castle Ravenloft?

We all know that if a vampire is reduced to zero health, they are forced into mist form and must make it back to their tomb within 2 hours or be destroyed.

How much of Barovia is too far away from Strahd's tomb for him to make the mist form trip in 2 hours?

And he is of course aware of this vulnerability, so under what circumstances would he ever let himself be drawn into combat farther away than he could make it back in time?