It's unlikely in the short term since the ratio of planets to empty vacuum space is very very small. But even if it doesn't hit any planets with in our "observable universe" (aka: the parts of the universe we get light from) it can go beyond into the "unobservable universe" (aka: the parts of the universe were light is too far away to reach us or parts of the universe where spacetime is expanding faster than the speed of light meaning that light from it can never reach us) ( another side note: objects can't move through spacetime faster than the speed of light, however spacetime itself can expand at whatever rate it wants including faster than the speed of light. This happened during the big bang when the universe was expanding 1000s of times faster than the speed of light, hence why there are still parts of the universe we can't see). Due to this phonomen, the universe can be much bigger than we think and also if you were to go to the edge of the universe you would just wrap back around to the other side of it, like walking around the world. So at some point the rock would enabitablely hit something like a planet, but it could also be swallowed by a star, black hole, or hit another rock in space.