While playing on SMP, was knocked into lava in the Nether. Wearing full Diamond gear (each piece with several mixed Protection enchantments, with one piece of Fire Protection IV), it took 20-30 seconds to swim to a ledge, however with the protections, I took minimal damage. After exiting the lava, however, the persistent fire caused damage at a higher rate than the lava. I've further tested this on SSP in the Overworld and Nether, in the Overworld both damages seemed to occur at a slower rate.