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A Steam Mephit has the following ability called Boiling Rain:
Once per day a steam mephit can create a rainstorm of boiling water in a 20-foot-square area. Living creatures within the area take 2d6 points of fire damage (Fortitude DC 14 half; caster level 6th). This ability is the equivalent of a 2nd-level spell.
How long does this last?
Despite the flowery description, the spell-like ability a steam mephit possesses to "create a rainstorm of boiling water" is an effect that deals its damage just once then the effect dissipates, like a low-powered, different-flavored fireball spell. Its duration is instantaneous.
Even if the GM were to rule—based on the spell-like ability saying that the mephit creates the rainstorm—that the rainstorm is a conjuration (creation) effect with an instantaneous duration (which it very well may be!), the rainstorm is neither creature nor object, and it would disperse after the effect concluded.
The alternative would likely see steam mephits make everlasting pockets of boiling ranstorms across the cosmos at a rate that would eventually doom all but them and other fire resistant and immune creatures. I leave this plot here for any GM who wants it, but that GM should be aware that the Improved Familiar feat allows even a PC to pick as a familiar a steam mephit.
Note: The steam mephit is ported pretty directly from the D&D 3.5 SRD here. The latest edition of that game's Monster Manual (2012) doesn't update this spell-like ability with a duration, making its duration instantaneous in that game as well.
It does the damage in the round that the Boiling Rain comes down. The DM could rule that it's still wet there, but the heat of that boiling water is expended, now it's just a recently rained on, slightly warm, piece of ground.
