Until they realize you just gave them 2 cups of dog food.

We often buy too much candy. When we realize we did, we usually have a soulful moment where we need to decide how much candy we're keeping for ourselves. Yes, we can buy candy anytime but we're adults so we usually don't buy bags of little sugar pucks.
After we've broken our hearts taking more than we need and less than we want, we usually try to wait until it's the last kid of the night who's old enough to understand but young enough to be utterly bombastically awestruck and dump some preposterous amount of candy in their bag after doing a quick non-verbal "you cool with this happening?" Negotiation with their parent.