The Season 2 Conquest Reward schedule
lists 25 weeks of rewards. As Season 1 lasted 20 weeks, a 25-week season doesn't seem unreasonable. We appear to be deviating from the Legion Arena season schedule, in which seasons occurred more frequently than new raids. With the ongoing polish to WoW's Esports structure, now all competitive forms of content update at the same time--raids, PvP, and Mythic Plus.
If we count 25 weeks from the start of Season 2, January 22nd, we reach July 16, 2019
for the start of Season 3 Battle for Azeroth, new Azshara-themed Mythic affix, and new raid, Azshara's Eternal Palace.
This would also align well with the recently-revealed WoW Esports calendar
, in which there are two seasons in 2019 for both Arena and MDI. A new season starting in July evenly splits the year in half.
The one awkward part is that Season 2 would be split up by system changes with the introduction of Patch 8.2 Azerite changes and new traits. This could affect the competitive balance, similar to the Patch 8.1 class changes that affected the Season 1 meta in December.