Presuming you're a DM, I'd say its up to you, and you should scale it to the approximate level of the party.Keep in mind that there's lava in many different forms - if you have a low-level party, they're fighting near a cool, slow-flowing, almost-solidified lava flow .. falling into it hurts, but it doesn't kill instantly.
If you have an epic-level party, they're fighting near a high-heat, fast-flowing lava flow ... falling into it does enough damage to kill a low-level character, but due to the heroic nature of the high-level character, it doesn't kill them immediately.
I'd refer to you to pg 42 of the DMG; you might find a low to medium limited-damage expression appropriate for "lands square in the lava" or "one-time effect from a trap", and a low normal-damage expression appropriate for any "save ends" effect.
I would also reccomend lava as more useful as a terrain feature than as a trap, per-se ... or if you're going to use it as a trap, make it possible for the PC's to also use the lava against the enemy after they've been hit. Encounters are a lot more fun when your shifting, pushing, and immobilizing powers can be used to push the bad guys into lava!
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