Given average interstellar density of hydrogen, how long before a one meter sphere of iron travelling at relativistic velocity erodes away?

Why?

Ever notice one of the standard weapons in interstellar war is the railgun, firing a stream of iron spheres really close to the speed of light? Not many contact something like a spaceship. So, what happens to the spent ammo, littering the universe? Eventually, the interstellar medium erodes it away, I'd just like to guesstimate how many lightyears that takes.

Think: as the iron is ablated off, it forms a fast-moving nebula of iron vapor. As each cannonball is in turn ablated, you can get a pretty decent wavefront of fast-moving iron vapor. It's too bad that Earth can be in the way of this.