I think it is a little less than 3 gallons total.
DEF usage varies greatly, and with the new "less cheaty" software, it stands to reason DEF usage would be greater.
Most of the 2012-14 CKRA Passats could go the 10k interval before, but some just barely. No idea how they will do now, nor how much different the 2015 (different engine) would be.
And it can vary depending on ambient temps, too. So until you have some driving under your belt it will be tough to say for sure what your particular car under your particular driving will do. So until that time, it may be wise to at least keep a small unopened container of it at home inside (the DEF is supposed to be stored between 12 F and 86 F) o top up in case the warning comes on before the car is due for its normal service.