A person can strongly believe that they have some enchantment on them, such as immunity to fire or immunity to physical harm or anti-magic aura, and thus actually gain that property. The effect is ongoing in the background, as long as the person’s subconscious believes it to be true. However, it is much harder to convince oneself of an enchantment that would be proven false on a regular basis (such as immunity to physical harm), since when one is trying to tell himself that one inevitably recalls all the instances where that hasn’t been true, and the lie will fail to take hold. Also, greater-magnitude/stronger and/or more complex effects are more difficult to take hold. The enchantment will take hold until perhaps some perceived life-changing or fate-altering event affects the person, in which case the subconscious might think that past enchantments no longer apply. Enchantments will also break if they run up against a person who has a stronger faith in his own spells and the two effects are contradictory. This would result in a conflict of will, but the two of them won’t recognize it as such, and the contest would last only an instant before the stronger side wins out. Generally speaking though, usually these effects can last for as long as the person is alive.