This has been tested with sorcerer, cleric, wizard, ranger and blackguard.
My seconds are exact enough to testify that its not working as intended, give or take one round. When I say 1 minute 10 seconds, that means its either 1m 6s or 1m 12s.
I don't know when a round and the timer is exactly iniated. Is it the summoning round or does it wait for the next one? It doesn't really matter for this at all since the duration far less than it should be.
Anyone can easily replicate these results. You don't even need AoE spells.
Another way is to attack the summon your self, or have it attack you.
In the first case, it will unsummon as fast as if it was in an AoE.
In the former, it will unsummon at the same rate as if it was bashing a door.
Both times far bellow of what it should be.
My impression is also that summon times out faster when there are several enemies attacking it at once, but I haven't timed it to confirm.