You find the canonical answer for this with the discussion of Thomae's Flying Castle:
We also discussed this recently here. In general and unless stated otherwise with the rules, as is the case for T: Circle, a spell's Target parameter does not interfer with that spell's Duration parameter.
It can lead to some interesting effect by the way:
The mage is in a location without clear marker, he cast a simple CrHe or CrTe with duration of Diam to create a room, then cast the main spell with Room target, with a much longer duration. 2 minutes later the room disappears, but the main spell remains in effect.
It is clearly spelled out "A spell with target Room is useless if there is no Room containing the targets; it cannot affect a "roughly room-sized volume" in open space." (p113, core rule)... so doing that way allows magi to have lasting effect despite the apparent lack of room equivalent.
And for those lacking skills or having defficiency, I am sure that a Verditus can craft within a season a minor magical item with the appropriate effect.
If your Target no longer exists, then the spell ends.
The Target can be damaged and altered, and the spell will continue, provided it is still a valid Target. So if you change a plant into an animal and then cast an Animal spell on it, the Animal spell will end when the animal reverts back to a plant. In the same way, if a spell affects a person's arms and they are destroyed, the spell ends.
It is hard to determine, just when "a Target no longer exists". And the proviso of "valid Target" is even more tricky.
It can be logically impossible for a spell effect to still ever affect the destroyed or altered Target. Doublet of Impenetrable Silk is by all means ended, if the cloth vest it once affected was burned to ashes and scattered.
You do not even need to consider any more, just when that effect ended: it is purely academic.
Or it can be logically possible, but the spell, if cast on the altered Target, would not have affected it.
By ArM5 p.92, a Bjornaer in human form affected by ReCo Curse of the Unruly Tongue cannot just change into Heartbeast form to lose that effect, though in that form the ReCo spell would not have affected her.
In such cases effects run for their duration.