Mark, as far as I know, there aren't specifics to be found on this subject other than "follow the instructions". As you may know, the labeling instructs one to discard any unused solution. But I will admit, I don't always do that. ;)

It depends on how long until the next spraying. If it's going to be in just a few days, and there's a fair amount left, I'll reuse. If it's at the end of the growing season, and won't get used for a few months, I discard. Judgment call/up to you.