I heard a fellow student in the Hunyuan Tai Chi class I attend started to feel a warm energy ball in his Dan Tian area (lower stomach) after about 6 months of practice. He is more advanced than me, so he might have done other internal arts before studying Tai Chi. But that's the anecdote I can offer. Speaking of how "Chi" is felt in your body - my teacher said it feels like a warm stream of energy flow. I am at a stage where I can feel heightened sensation in my palms (like being gently needled), but obviously they are in their early development stages. Just to explain how we are trained - we have to do Qigong in addition to the form exercises. If your training does not incorporate Qigong or Neigong (internal kung fu), then it might take a very long time to develop any feeling of Qi, if you only practice forms. Without Chi, you are only doing Tai Chi dance, not tai chi martial arts.