The mania of Dec 2017 and subsequent fall is starting to look very similar to what happened in late 2013 to early 2014. That was about a 90% correction. If this same pattern holds, then we could see retracement values on bitcoin to go as far down as $2500 for the lowest low over the next 2 years. That's about as bearish as one can get. This assumes that the situation is equal to what happened back in early 2014. I don't think it is. The network is worth much more now.
I expect the next mania phase to begin summer 2020 to peak like it did in Dec 2017 again by Feb 2021. I wouldn't be surprised to see bitcoin crossing the $500K mark by then.