That totally depends upon how long you are actually leaving it for once opened. Realistically, it’s unlikely that you are going to significantly affect the alcohol percentage itself as there won’t have been enough time for the wine to evaporate. It could take weeks for this to happen. However, once that cork has been removed, other physiological changes do begin to occur that will affect the test, not always in a bad way either. If you’ve ever been on a wine tasting evening, you will know that tannin-rich reds need to air. The process that’s referred to as “letting a wine breathe” can temporarily soften and improve the taste of some red wines and it’s actively encouraged that you decant them and air at room temperature before guzzling! Eventually, though, air feeds bacteria which will make your wine taste different.\r