My handy US Army Survival Handbook recommends 1 minute of boiling at sea level, adding 1 minute for each additional 300 meters of elevation, or just boiling for 10 minutes wherever you are in the world.

I've heard that water purification beyond filtering, using chemical tablets, or simply bringing water up to a boil then stopping, is usually overkill here in the states. The nasties that aren't killed by these methods are usually only present around third world countries. I don't know how true that is.