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.
With the current trend I doubt Ethereum would bypass and be the first one before the bitcoin.
Ethereum is way too far from bitcoin and I think that in many years from now it can overtake the first position.