Actually, Ren's link has charts with temperature and duration. I can't claim to be an expert, but this looks like credible info to me.

Personally, any "extra" boiling after the 1-minute mark depends on the water source.

For a glacier-fed mountain creek, achieving a full rolling boil is plenty -- I don't waste fuel.

On the other hand, if the source is the Amazon or equivalent, I'll boil the devil out of it if I can spare the fuel. I seem to recall that there are lots of spore-stage nasties in tropical water that are tough to kill.

Same goes for a water source with cattle in the area. Cattle are notorious for walking knee-deep into a water source, drinking on one end and simultaneously dropping cow-patties from the other -- ground zero for a huge e-coli load. Boil, baby, boil -- with some campfire charcoal if possible.