Growing Sweet Potatoes

I have started the process of growing sweet potato slips by submerging half a sweet potato in a jar of water, with the other half sticking out the top. After weeks and weeks of nothing, it has finally grown a very good root system. However, there are no signs of any slips starting to grow at all! Has anyone else had success at growing sweet potatoes this way? How long should it take for slips to start appearing now my sweet pot has grown a root system?