Fragile aspects, by the RAW, have to be used right away and go away once used- though your GM may rule that sometimes, as appropriate, they'll last longer:
You perform a maneuver to <Blind> an enemy using your flashlight (single use compel tag), but then don't use it for whatever reason. There's no reason why 13 rounds later, he suddenly suffers as a delayed result of that. Use it or lose it.
You perform a maneuver during a downtime (at the beginning of a session where a month or more has passed between sessions), describing how you've been saving money up for an <Emergency Fund> (single use aspect tag for a later resources roll). There's no reason why this would vanish if you don't use it right away, so your GM (if he's nice) may allow it to stick around until it comes up... if however, it doesn't come up that session, it may go away next downtime (you ended up needing it when your car broke down last week).
The number of shifts a fragile maneuver costs is equal to the strength of the maneuver- this goes for both magic and mundane skill rolls.
Sticky aspects (can be used more than once- only the first use is a free tag, costs FP after that), can last longer, but again, only as long as appropriate, and generally never more than a scene.
The number of shifts a sticky maneuver costs is equal to the strength of the maneuver + the number of extra uses you want to have available. For your question- the reason the evocation spell lasted for a scene is because it's a sticky maneuver, and that's how long sticky maneuvers last.
If you're using evocation, that's the limit, I'd say. If you're using thaumaturgy, and you want to make the maneuver last longer, you can probably pay for extended duration, but it might be more appropriate (at least, for aggressive maneuvers, not self-maneuvers) to try to inflict a consequence of the appropriate type/duration.
