Can I use a store-bought guard?
Sometimes, but a custom guard usually fits better and can be designed for your specific risk.
Is this the same as a night guard?
It can overlap, but sports guards and night guards are designed for different forces and use cases.
Can it help with TMJ symptoms?
Sometimes a splint is part of TMJ-related care, but the bite and symptoms should be evaluated first.
Longevity depends on use, grinding intensity, material, and maintenance.