At this point the specification has become a Candidate Recommendation and so is not going to change hugely at this point. There are very comprehensive implementations behind a flag in Chrome, in WebKit and in Firefox. When grid is shipped in browsers it is going to be shipped in a very solid state cross browser, so there will be no reason not to start using it with appropriate fallbacks for older browsers.
CSS itself has built in support with Feature Queries that will mean we can write code for browsers without grid support and progressively enhance for those who do. So I dont think you will need to wait until 2018 assuming we see grid shipping in early 2017. As with everything though that will depend on the profile of the people who visit the sites that you build, and the kind of designs you are implementing.
In terms of learning however, you won't be able to make the assessment as to whether you can or can't use Grid if you haven't yet learned it, so it is definitely not too early to start to learn how it works. I've provide a whole heap of resources, including videos over at http://gridbyexample.com.