So... as a refresher 8.1.5 comes with:
Kul Tiran human and Zandalari troll allied races
Darkmoon Faire rollercoaster ride
Children's Week update
New Micro-holidays: Wanderer's Festival, Vash'jr Diving, Free T-Shirt Day, etc.
Warlords of Draenor Timewalking
Portal rooms in Stormwind and Orgrimmar
Tools of the Trade profession questlines
Return of Brawler's Guild, including a new murder mystery questline
Remastered Warsong Gulch and Arathi Basin
New PvP Brawl: AI Comp Stomp
Which means we have at most until late April to wait until its release, since Children's week begins May 1'st. Hopefully it'll launch before then. I reckon we won't see it until at least the final wing of LFR for Dazar'alor launches.
Maybe we'll see an 8.2 release in July?