I have a 2016 XE AWD, I usually wait until a low fluid warning comes up on the screen then pour in 5L of DEF fluid, ( £6.00 for 5L from Asda ) the 5L lasts a long time, I have never had any problem.
Had my 2016 XE for three years now, love it, even after numerous attempts to update the sat nav it still thinks I am crossing the river Forth by ferry boat rather than the Queensferry crossing.