it is general knowledge to decanter a complex red wine,with white wines the thing is more tricky and depends highly on the thing in Your glass.
Decantering too long(8 hours and more for young ones) makes the wine softer,more accessible but less descernable in comparison to others,so don`t decanter too long for blind testings.
With a great wine,follow a simple rule,even a 3 hours and longer decantered young wine will improve in the glass(mind You have the proper one!!),I often made the mistake in testing to empty the glass(filled 1/16 l)within 10 minutes,and after 30 minutes the remnant wine "came"(especially Latour and Lafite need a lot of patiance),for Mouton frequently only the dépôt-degustation next morning offers the full pleasure(as Thomas wrote above),same thing happened to me numerous times with LePin and Petrus.
Needless to say You have to be very careful with vintage bottles 40 years and older,these old wines must never be decanterd too long and only the best ones
improve after 5 or more minutes in the glass.