Yep, despite the cutscenes suggesting it's urgent, the defenders are on the verge of being defeated/the invasion force is on the way/whatever, there's nothing stopping you from spending years in-game going around and rescuing artwork from religious mobs, escorting corrupt politicians, punishing defaulting vehicle owners, supervising new militia recruits, assassinating obnoxious diplomats, etc. while the Restoration patiently waits on the sidelines.
A quick note: after you are done with the main campaign (or if you start a new "career mode" as opposed to "campaign" playthrough), you'll see special missions called "Flashpoints" pop-up. These are timed (and the time limit will display in the description), but will go back "in the stack" and a new one will be drawn when they expire, so the expired one will eventually pop-up again, it may just take a while. Again, this doesn't effect you during the campaign, because the campaign "priority missions" never expire.