Until when ethereum will continue to decline, will it touch at the price of $ 50?

It shouldn't have fallen like this.
December this year is very different from December 2017. The situation is reversing.

The declining of Ethereum will continue to happen in December due to the depression of the market. The opposite of the last year, when the ETH increased and reached a peak in December. So that I think you will wait for the recovery of Ethereum is predicted to recover in 2019 instead of 2018 because the low possibility to the market to rise at now.