How far north does one need to go to see them?
It entirely depends on how far south they come, which in itself, is due to a combination of the strength of the CME (Coronal Mass Ejection) that strikes the earth, the direction the earth's magnetic field is tilted at the time (south is favorable for northern lights, whereas north inhibits them), how far in that direction it is tilted, and whether the CME is directed squarely at earth.
Normally, in our part of the world, most displays of northern lights are visible only well up into Canada (with the top end of the swath beginning at about 20 degrees south of the north magnetic pole, which is currently somewhere on Ellesmere Island). However, sometimes they are visible into southern Canada and the northernmost U.S. states, and less frequently, as far south as the southern states, the Gulf, Florida, or even Southern California. On the rarest of occasions, they have even been seen in Mexico.