Crucial point. After all, if you can't just sling it, it is, for instance, no good in combat. Which is good for some stories, not so good for others.
On the other hand, given quickly cast spells, how long do they last? If you can do something that lasts forever but is cast only in five seconds, the imbalance seems extreme. Limited use setting, with spell piled on spell itself piled on spell. . . .
But if you have spells that come and go like fireflies, that's also a limit. One might light a fire that way, or cure someone -- since you are removing a problem -- but you could not, for instance, have a permenant enchantment that could be stolen.
A variety of lengths probably requires a system to hold them all.
Alternatively (you knew this was wrestling with a problem didn't you?) I make the burglar steal something NOT for its magical properties. Or possibly for its uses in magic, not its inherent magic. The problem there is avoiding a pure MacGuffin. . . .