How much of your grocery shopping ends up straight in the bin, on average?

A friend of mine throws away a fair bit of his, usually due to forgetting about it and it going off, with his reasoning being that it doesn't really matter if he eats it or whether it goes in the bin. This irks me somewhat.

I try not to throw anything out, even if it's the remains of whoever I'm eating with. I'm very greedy. Would I be a greater strain on resources if I continued this and became obese? Who knows.