Mantras are based on intention. You’re trying to program your mind to do something later, whether it’s to perform a task, remind you of something or anything else like that. For instance, thinking to yourself “I must remember to pay this bill later” is setting the intention to pay that bill. Most people will then remember at the appropriate time to pay the bill.
Whatever you say, whatever the length, it’s the intention that matters. If you haven’t already, I would recommend you read BenDrummin’s article Commanding the Subconscious Mind.