If you are planning to migrate from an m-dot site to a fully responsive website then the time to do so is now, or at least a before the mobile-first index is rolled out in early 2018.
In a recent Google Hangout session, John Mueller said that anyone seeking to migrate their site from a http://m.domain.com layout to a mobile-friendly responsive site that can adapt to any screen size should take the required steps prior to the mobile-first index being introduced next year.
The reason for this is that, up until now, Google has always used the desktop version of a website for indexing and ranking purposes, without taking too much interest in separate mobile versions of the same site.
However, when the mobile-first indexing system comes along, the m-dot version of a site will be indexed in the same way that a standard desktop website is crawled and cached at present.
This means that any redirects, or any additional migratory changes which involve moving canonical pages to updated URLs, may take a good while to be re-indexed and updated in Google’s database when the mobile-first index is finally up and running.
By migrating to responsive now, webmasters can avoid any unnecessary delays and transitional disruption by having the latest version of their mobile friendly site already listed in the mobile-first results as opposed to being forced to wait for their new URL’s to be crawled, ranked and re-indexed by Google’s algorithms at a later date