I've gotten about 2 weeks out of my batches on my 5 gallon. Just started up the 10 gallon, but expect a week or so. I don't have a way to accurately measure output, but the bubble counter is about 1.5 bubbles a second, and the drop checker is showing a nice green.
I get my citric acid from WalMart. Each batch I make takes just about 7 oz of Citric Acid powder (200g). It's in the canning aisle, and costs around $3 for 7.5 oz of powder. It's a little more pricey than the link above, but no shipping costs. I have also seen it on ebay, and there a couple of sellers that offer free shipping. I may go that route next, but it's pretty convenient for me to just pick up a few jars when I am shopping.