Fron my experience yes.. They do wear off. The more energy you put in, the liner it lasts.
If you feel like the spell accomplished what it was meant to, then you probably don't need to upkeep it.
Meaning: if you cast a spell to keep a bully away and they stay away even after you feel the spell has worn off, then your will has been done. Goal achieved.
BUT if you feel like this person may become a threat again then by all means maintain that spell or at least carry some sort of protective talisman.
Go with your intuition.