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.
Actually, splitting is one very convincing reason for a lot of people to buy Bitcoins. As a matter of fact, there is already a rough pattern that somehow suggests that whenever there is a split that is about to happen in Bitcoin people tend to dump their altcoins and transfer to Bitcoin. Well, all because there is a free coin right after that.
Ethereum will not be able to pass up Bitcoin. The market is the hardest proof of it. Take a closer look at the market right now. Ethereum is having a harder time than Bitcoin.