Well you'll have plenty of time to get to know them before the DLS's even on the horizon if you're picking it up close to launch. They say that they're not even going beyond planning stages until XV proper is released, so if the way they're listed here is the intended order of release, I'd bet we get "Booster Pack" at or extremely close to launch, and "Episode Gladiolus" sometime maybe early-mid December around the "Holiday Pack."
That'd put Ignis and Prompto in 2017, maybe something like February and April/May, and then "Online Expansion" probably Summer 2017.