You need over 5K hits from your skeletons to get to level 50. As you progress through the 30's and 40's you can find yourself spending days parked by greydwarf spawners but using the resting bonus can significantly improve that. But if using the resting bonus be prepared for events. While increasing my blood level I had two troll and 1 seeker event. By that point my skeletons had enough skill level to actually take on the seekers. Oh, remember Ulf? Well I guess he didn't make it. But his skeleton does a fairly decent job with a sword.