I thought for sure it would take until the end of 2018 but it looks like this summer ethereum might pass it up. With the turmoil around BTC potential splitting, it might be very soon. IMO this is healthy, we will always value bitcoin for being the one that started it all, but it's time for a more dynamic and adaptable currency to take the lead.
In terms of market capital, there is no currency in the encryption field that will challenge Bitcoin, but for some time I think that ETH will exceed the price of Bitcoin in terms of market capital, but from this point, Bitcoin has begun to rebound it with There is a huge gap between the rest of the coins. Bitcoin will always be number one, Ethereum has no chance to chase Bitcoin now