Relatively little. At 40% ABV the water in tequila evaporates at about the same rate as the alcohol (which is to say not particularly fast) so as long as there's some liquid in the glass it should taste about the same (or better--most aged spirits are much better after breathing for at least 10-20 minutes). In a high proof tequila the alcohol will evaporate a little bit faster, but still not enough to be meaningful over the course of an hour.