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.
It is difficult to grasp for me why people always compare BTC to ETH. Both are entirely different, one is finite in number, the other has endless supply, it is way too easy to get an idea what would happen in the long run. There is a simple logical reason why bitcoin is called digital gold. Secondly Ethereum is a smart contact based development platform, Ether the currency. Compared to bitcoin client, the scripting language of Ethereum is susceptible to hacks and attacks, DAO hack.
Comparing BTC to ETH is like comparing an animal to a bird, both have different working models. Ethereum has the potential to give this world many innovations which would make it a better place. But if you are going to compare Ether with BTC as a currency, Ether would not stand a chance against BTC in the long run.
Yeah, the scaling issue is causing some problems right now. But it would be fixed. If BIP148 does not split the chain and Segwit gets activated we would have a solution within the next few months or most probably by 2018, Segwit2x.