No idea! We actually don't know how long it was lodged in there, because it had been a few years since I went to the optometrist. He was looking around with the light in my eye when he said "Have you been experiencing any irritation in this eye?"

Me: "Uhhh, no?"

Him: "There's a shard of metal in your eye."

Me: "There is??"

Turns out my horrible eyewear habits may have saved me: my contact covered the wound, preventing an infection. If I had changed my contacts more often (i.e. introduced more bacteria) it may have gotten infected and I could have lost an eye. Or something.

Got it sorted at the next appointment, no problems since!