I think the time is right to invest (at least much better than December 2017), after such a fall we can assume that the market will soon stop falling.
The best is to invest in steps, it reduces the risks (to smooth the purchase prices). Anyway I think if the market continues to go down much more, either the crypto is completely dead or the market will take years to recover (and so it will be necessary to return in several years).
If the market will start again, the rise could be rapid and it would be a shame to miss it.
 
Provided you choose your projects carefully and don't invest in anything.