Just wanted to clear up some confusion with regards to what "half-life" means. I noticed there was an error that wasn't corrected so maybe not everyone understands.
Basically, you started right.
Half life is the amount of time it takes for one-half of vitimin C in this case, to decompose. You are then left with one-half of the original. Then, one-half of this amount will then take that same amount of time to decompose and you will be left with one-half of that amount. Mathematically, you will never reach 0 but at some point it's basically gone.
So, as an example, let's use the vitimin C with a 2-hour half-life (I'm assuming that's right as I didn't look it up.)
Take 4 grams (starting point). In two hours, there will be 2 grams. In four hours there will be 1 gram. In 6 hours, there will be .5 grams. In 8 hours, there will be .25 grams. In 10 hours, there will be .125 grams. In 12 hours, there will be .0625 grams. In 14 hours, there will be .03125 grams, and so on.
4, 2, 1, .5, .25, .125, .0625, .03125
Another quick approximate example is Plutonian 239 which has a half life of about 24,000 years. So in that time, half will be gone. Then in another 24,000 years, about 1/4 will be left. Then, 24,000 years again and 1/8 will be left. Then, 1/16, 1/32, 1/64, 1/128 and so on.
To reiterate, half life does not mean half is gone in that time and the other half is gone in double that time.
Hope that helps.