I keep LME for 6 months usually. You can keep it longer but it might get mold on the surface. You can just dip the mold off and still use it(you are going to boil it anyway, so you will kill any germs). I buy 33 lb jugs from Alternative Brewing in Charlotte for $59. Leisure-time sells 33lbs Muntons for $46.79. http://www.leisure-time.com/muntmalt.htm
DME will store indefinitely as long as you keep it dry. It costs way more then LME. I think the best deal I have seen is Leisure-time above....they sell 55 lbs of Muntons DME for $147.
LME = $1.42/lb
DME = $2.67/lb BIG Difference!!!!!!!!
For those that worry about the hassle of weighing out LME.....I just use a Half Gallon Milk jug. When it is full it weighs @ 6 lbs.