It would really vary greatly for a 400m as opposed to a 1500m runner. A 400m runner probably spends <20s or so in lactate producing state. A miler on the other hand might be there for several minutes. Also, you'll find most coaches (at least the ones on this board) don't focus 400m training around metabolic concerns as much as neuromuscular considerations. For the 800m on up, metabolic issues play a much greater factor.