I think it all depends on the chins. My 2 boys will eat 2-3 hands full of timothy a day, my 2 girls will eat a lot less. I also offer them alfalfa cubes, but usually they will just nibble on cubes and leave the rest of it there, a few times I found the left over cubes in their litter box, I think they were trying to tell me something..LOL~
You can buy a small bag of timothy hay to start, and see how much your chins will eat per day, then go from there. Also, store your hay in a cool dry place without direct sunshine will help keep it last longer without going bad. Good luck. =^_^=