Any realm/domain can be assumed to have a system of dispatch riders/pigeons/ravens/magicians/whatever that will enable them to effect semi-fast communication within their province/holding network (real wizards can make it go even faster as Regien so wisely pointed out).
Assume that within the span of a single week/war move a dispatch rider/whatever can move hundreds of miles - assuming good weather, friendly territory etc. Not exactly instant, but not so slow either. But then figure into this that someone has to observe, think, discuss, write, and then send...and then there is bureaucratic reception in the other end...
Let's say that you can respon more or less instantly to things taking place in the same province as your regent, within the week if within a province or two (under the assumptions made above), and beyond that considerably longer.