According to an interview with Folco Galli, the head of information at the FOJ, on the radio program Info 3, the revised DPA is planned to enter into force ideally as early as August 2018:
As expected, the draft and message are to be presented as early as August 2017. The bill could then be discussed in the 2017 winter session or the 2018 spring session, so that the referendum period would run until mid-June 2018. This would allow the new DPA to enter into force as early as August 2018, if no referendum is filed.
The interview is available here (from 7:10 min):
Download: podcasts.srf.ch/world/audio/Info-3_30-06 – 2017-1200.1498818184829.mp3.