Dave’s advice is sound Boadle, model one story and unwrap it, then clone the geometry as many times as you have floors in your building. You can still weld up the verts to make it one object, and all of the UVs of each floor will be shared so you should get good texture resolution on each floor. If this is for real time purposes you’ll want to watch your texture resolution so be aware of this if you’re using render to texture. If you want to give variety to a few windows you could always place a quad over the area and unwrap that seperately to place the texture variation on it.
Alternatively, if your building is the same shape the whole way up you could try tiling a texture on much simpler geometry to achive a similar effect and maybe save on polys??