When will the maps be changed, due to continental drift ?

Continental drift is incredibly slow. Tectonic plates move a few centimeters a year. So, let's assume one continent, moving the maximum in the normal range (7 cm), all in the same direction, (which almost certainly wouldn't happen), for 1,000 straight years. That continent would move 7,000 centimeters. That's 70 meters, or 230 feet. Less than a football field over the course of a millennium.

Now, does that mean that our maps will never need to be changed again? Absolutely not. First off, the political borders are changing frequently. Historians will make use of our current maps the same way we use maps of pre-WWII Europe to understand how things were set up then. In fact, they may make their own maps of our time that are more accurate than our own. For instance, a political map of the current world made today would show the Ukraine as including it's eastern half. Future historians and mapmakers may, depending on how things play out, show it as already having effectively broken off into it's own country.

But if you're just talking about physical, non-political maps, there's still need to change maps frequently, even in Continental Drift isn't (directly) the reason. Take the recent Nepal earthquakes. It probably only moved the continent a couple centimeters, but Kathmandu itself moved 10 feet. Now, your normal paper maps will probably not need to adjust anything for that. But Google Maps? or Bing, MapQuest, or whatever local service they have in Nepal? They do need to adjust, especially if they use GPS. 10 feet is still not massive but it can be the difference between being in one lane or the other, being inside a building or out. So that kind of physical map does have to get adjusted for local areas affected by big earthquakes.