How long do one time use wondrous items last in rounds?

[setzer9999]

Example: elemental gem (water)

It summons a large water elemental... but for how long, and why that long, if you could help me out with the "why" part too so I may understand the lengths of other similar effects.

[Quantum Steve]

Elemental Gems summons an elemental as per the summon nature's ally spell. The duration of spells is based on the CL of the caster. In the case of magic items, we use the CL of the item, in this case the CL is 11. So the spell lasts 11 rounds.