Are you asking me to do long-term software development estimation? 
Everyone please save this post so that over the next few years you can periodically give me back exact quotes demonstrating what I’ve gotten wrong:
Given the current status and level of community contributions I expect that ROS 2 will be well-featured and very usable for a wide variety of applications long before Melodic goes EOL in 4.5 years. At some point between now and then we’ll get to a situation in which any new robotics project that could be tackled with ROS 1 could just as well (and hopefully better) be done using ROS 2. By that time it will also be feasible to transition any existing ROS 1 system to ROS 2. And before then we’ll see new systems written from scratch using ROS 2 that are deployed in products.
Of course, irrespective of the state of ROS 2 I expect that many many people will still be using ROS 1 in 4.5 years and even after, primarily because of a combination of familiarity and reliance on existing ROS 1 code.