there are a number of things that affect the answer to that question - Cartography Research, Lighthouse building, World troop speed and direction of travel. Best way to find out is to look at the map and pick an island far away and see how long it takes; either by mocking up an attack (you don't need to have a CS built to check) or by using the troop drop down in the upper right hand of the main screen.
I'm in Gamma, have no research or light house and the travel distance limit in a straight line up from me is about 240 coordinate points.