As soon as is better to buy ETH because if we compare the price among recent months, the price of ETH is continue getting high so my respect advise is buy it soon otherwise we will lose the chance. And if we go back to 2017, the price of BTC and altcoins including ETH is high in the middle of that year till the end of  2017. So, it is probably still will be going to high till the end of this year.