Depends on the wine. The purpose of letting one breath is to oxidize the tannins and bring out the aroma and flavor. An hour is good for many reds. More than four hours is usually too much; the wine will degrade, losing it characteristics and becoming good for nothing but cooking or making vinegar. It varies a lot, though. Simpler, younger wines typically benefit more from breathing than do mature, complex ones. In fact, you can make a cheap wine taste like one of moderate quality by blending it with another compatible one, and letting it breath for a couple of hours before having any. Just remember not to let any wine sit out open too long. If you don't drink it all, use one of those rubber valve corks and a vacuum pump to make it last another day or two in the refrigerator.