For instance, I have some bars of Valrhona I use for chocolate tarts and pastries, but I haven't been doing the dessert thing in a while. How long does chocolate last before losing flavor (or does it)? And once it gets that white stuff on the outside, is it done? What's the best way to store it for as long as possible? Thank you.
Regardless of type, all chocolate should be stored in a cool and low humidity (dry) place away from direct sunlight. It would be best to seal it in an air-tight container, because, as ElendiTheTall said, the cocoa butter in it will absorb flavors.
Dark chocolate will last for years. Milk and white chocolate will last for a much shorter time (a few months), because of their milk content.
Improperly stored chocolate will develop bloom, which shows as a white or grey streaking or spotting on the surface. The spotting or streaking is cocoa butter fat separating and is a sign that the chocolate's temper has been lost. This kind of chocolate is still suitable for any application where the chocolate will be fully melted (most baking). It can even be used as the base (non-seed) chocolate for tempering with the seeding method, but it should not be used for other candy making.
Stored correctly, chocolate can last for a very long time, due to its extremely low moisture content. Cool and dry are key. Tightly wrapped, it can be frozen, but this seems unnecessary.
4 months, 6 months is no problem. Years might start to be excessive, but in the end the chocolate will still be usable.
The true shelf life on chocolate depends a lot on how it is stored and what type of chocolate we are talking about. For instance, if a chocolate bar is stored in the pantry it will last for 2-4 months - but if stored in the refrigerator it lasts for 4-6 months BEYOND its printed date. The best way to store it is in a cool dry place like the refrigerator. Because chocolate has a low melting point it should not be left in warm places or in the sun for too long. You can freeze it, this lasts the longest!
The best way to store it is in a cool, dark place with low humidity. You should not store it in the fridge. Chocolate absorbs other flavours like a sponge so keep it away from anything with a strong odour.
The white 'bloom' on chocolate is the cocoa butter seeping out of the surface. It is harmless and will disappear when you melt the chocolate.