I've never experienced flash burns from a welding arc, but I've met several people that have. (Back in my Navy days, especially when we were in the shipyard...) They explained it as akin to having hot sand in your eyes and not being able to flush it out. (Burning, plus a scratch-like pain that won't go away.)
Doesn't sound like fun. (I understand it's basically a sunburn for your eye, caused by the UV of the arc... Right?) And they all said that the pain lasted for several days, though it didn't really hurt when they were being exposed to the arc in the first place.
Either way, I've always shielded my eyes around welders after talking with those folks.
Adam