It's been renewed at about the same time every year for three years now, and Season 3 will premiere at roughly the same time Season 2 did.
One of the show's biggest strengths is how well the story has been planned so far, with the plot never wandering aimlessly and plenty of foreshadowing, even in stuff that seems like gags or just background detail. Hell, they revealed that Queen Eclipsa is still alive just by having her in the background for a few seconds among a dozen other nameless villains and monsters.
On top of that, it's definitely not coincidental that right after the Season 2 finale they released a guidebook full of worldbuilding for Mewni and other information that has never come up in the show yet. We're not going to run out of story or content anytime soon; if anything we're only just now moving into the meat of it.