We did used to recommend that people take it for a week prior to the trip, but as time has gone on, that clearly was unneccessary.
I think most now talk about starting the night before (it is usually dosed as a single daily dose for most people, as that maximizes the effect on the nightime breathing problems.)
If you think about it, the effect is to accelerate one's acclimatization. It is actually hard to do that, until one is actually EXPOSED to altitude, although getting the chemical reactions going starting the night before seems reasonable.
I'm not sure that there have been any actual chemistry measurments of all this, though, but that seems to be the "best practice" currently being used by most.
