I do a standing version of a tricep extension.

http://exrx.net/WeightExercises/Triceps/DBTriExt.html

My question is how far is too far? Should I lower the dumbell as far as she goes, or stop at the point where my elbows are bent at 90 degrees?

I don't want to perform improperly and put too much stress on my elbows.

whatever is comfortable