It's fine. If your mind wanders off after 15 minutes then the idea wasn't that interesting anyway. There's no requirement in life to be more productive than nec

It's fine. If your mind wanders off after 15 minutes then the idea wasn't that interesting anyway. There's no requirement in life to be more productive than necessary. Spend your time to enjoy it and go wherever your mind wanders off to. Forcing yourself to concentrate is just sending yourself a message you needed to be forced to work on the idea, which is counterproductive if you want to work on your ideas. If the idea has potential, it'll come back, your mind will keep expanding on the idea, until it's interesting enough for you spend a whole day on it.