Hi,
If you would have asked this question before April 21st I would have said to publish the responsive pages the moment they were ready, even if the remaining pages are having a different design.
At this point, it seems that the impact of Mobilegeddon is quite limited, so if you can finish the complete site in a couple of weeks, I would rather go for the better user experience and present the full site with the new design.
If in the next days the mobile update would have serious impact on search results, you can always decide to publish the pages which are already responsive.
Keep an eye on this post - https://moz.com/blog/day-after-mobilegeddon - if the update really starts having an big impact, you'll read it there. (or follow Moz / Searchengineland / Searchenginewatch / ... on twitter & you'll be to first to know)
rgds,
Dirk