I use the whole bunch, but only the leaves. The more lime juice you add, the longer it lasts. One trick I use that stretches it out to about a week, is layering saran wrap for storage. Take one layer and mold it flat on top of the guac, leaving no air. Then a second layer on top of container to seal the whole thing. The less air it gets, the less oxidation. The brown layer is safe to eat, it's just not very appetizing.