Pellets should be about 1-2 tb a day, but that's just because that's how much a chin eats in a day, if your chin eats more give them more. You'll learn once you get your chin how much it normally eats in a day, my two for example eat 2-3tb each a day. You don't want to give more food then they will eat in a day or two since the food does slowly lose nutrients when exposed to the air. They don't normally over eat pellets so some people do free feed, just make sure they are eating their hay if you do. I prefer to measure the food so I can tell right away if they aren't eating.
Hay on the other hand should be free fed, it makes up about 70-75% of a chin's diet. It's easier to think of it as they eat hay with pellets to supplement it, since just like a horse, chins can survive on hay alone (though fur quality does suffer). There is frequently a good amount of wasted hay, chins are picky as to which strands they want to eat and wont eat hay once it's been peed on, so having fresh hay always available ensures they are getting enough to wear down their molars. (Chew toys wear down the ever growing front teeth, grinding up hay wears down the ever growing back teeth.)
How long you use the dust really depends on your chinchilla and how often you bath them. Dust can generally be reused for several baths so long as the chin didn't pee in it, just sift out any poops after each bath. Baths should only be given a few times a week (generally 1-3 times a week depending on where you live), so you don't leave the dust in the cage all the time. You can tell the dust needs changing when it changes consistency, going from fine and powdery to gritty with small clumps.
