Let's back-of-the-envelope this a bit... Let's say that there are 10 big interstellar rocks with a volume of 1 km^3 each that are currently within 30 au of the sun (roughly Neptune's orbit). If we assume this is a good estimate of nearby galactic space, then if you chose a random 1km^3 "voxel" in our neighborhood of the galaxy, the odds that it would contain a 1km interstellar asteroid would be roughly (10 * 1km^3) / ((4/3) * pi * 30^3 au^3) ~= 3 in 10^29.

Now imagine a spaceship that travels 100 light years. It's going to pass through roughly 10^15 1km "voxels" on its journey, which means the chances of it hitting one of those 1km rocks is ~(10^15/10^29) or 1 in 100 trillion. Space is biiiig.

The real danger, though, is the smaller stuff. It's hard to know the distribution of interstellar dust size, but there are a LOT more small things than big things, and it only takes a grain of sand at relativistic speeds to cause a really bad day. If sand-sized rocks turn out to be 100 trillion times more common than these big ones, well, that's going to be a problem...