The juice does oxidize, as juicing the goodness makes it very vulnerable that way. So a large percentage of the enzymes are lost in as little as 15 minutes. Still, the major nutrients (vits and mins) are still there for hours, so drink in whatever way is most comfortable for you.
Some people dilute their juices 30 to 50% to offset that power feeling. Try to "chew" at least some of your swallows; getting more saliva into the digestion process really helps. :)