Empirical studies yesterday evening showed that it takes a little less than two seconds to take out a sapper.
Not that I'm not grateful for getting some more-or-less free points there, but that spy (can't remember who was it) should really get his priorities straight when taking out a sentry with an engy on top.

I half expected him to get bored with that routine and just leave...
Witness the following hlstats shot by yours truly (as seen in my action log):