Ethereum overtook Bitcoin in terms of blockchain size in around February 2017, according to the stats, when both were around 130 GB.
Not just that, according to some comments on twitter the problem of Blockchain bloat would be compounded by the fact that the free-for-all nature on Ethereum-linked wallets meant that many users had a considerable number of them.
A wide range of altcoins are based on the Ethereum blockchain, many of which continue trading activity with the completion of various ICOs.
Continued upward trajectory could see the biggest altcoins blockchain hit one terabyte before 2018, making syncing very difficult to carry out.
I think 1 terabyte before 2018 is really hard to achieve. But I am also sure that ETH not having 1 terabyte blockchain before the end of 2018 is also impossible. Year 2018 will be crucial for ETH. Many ICOS will expect that ETH problems solve, but will just getting bigger.