You'd use 50 lbs quicker than you'd think. That's only about 5 batches of moderate strength, up to maybe 10 if you only brew lighter session beers (so call it 4-8 on average, depending on your brewing habits), assuming 5-gallon batches. My LHBS charges about half the price per pound for a 50-lb sack, and I figured out that I'd break even before the end of my third bag, so that's a year (if you brew a lot) or three (if you don't) of brewing.