10 seconds (actually 6) in lava with iron armor
With diamond you can get up to 10
Up to 3 can spawn in a vein (most of the times 1)
Smelitng the Ore leads a Vulcanite gem
10 seconds in lava with Vulcanite armor
EDIT: That's the old damage reduction values. With the new values in the Nether you would still have 6-7 hearts.