Titanfall 2 is doing something unprecedented in modern FPS games. They will not be charging you for new maps or modes, nor will they be offering a "season pass" for additional content. The team made the announcement via their website in which they wrote the following...
No Season Pass required: all maps & modes will be free in Titanfall 2 Multiplayer. This means no splitting up the community and it all starts with the legendary Angel City map, remastered from the original Titanfall. This means once you’ve purchased Titanfall 2, your investment includes a full single player campaign, the full multiplayer maps and modes, and long-term support with no hidden costs. You can pre-order the game to play 3 days early, but it will never cost you extra.
It would've been even better if they nicked the incentive to pre-order, but I understand the age of gaming we live in. Now considering they specifically mentioned maps and multiplayer modes, I don't think this game will completely free of DLC. I'm sure there will be alternate skins for Titans and pilots you'll be able to purchase or something, and if they expand the single player campaign I would expect them to charge for it.
Let's not worry about the "when they do" and just be happy that, for now, Titanfall 2 isn't immediately reaching for our wallets after purchase. Titanfall 2 launches tomorrow on all major platforms.