There's a wide variety of answers for how much warning. Most of what the Near Earth Asteroid Program or focuses on is Earth Crossing Objects or Near Earth Objects. Those pose the greatest risk of impact and they are also, somewhat conveniently, the easiest to track.
The hardest to track is a high eccentricity long period orbit that approaches from the sun side of the planet. Chelyabinsk was a sun side asteroid (an asteroid that hits during the day generally approaches from the sun side. Those are the hardest to see, and even though Chelyabinsk was too small to worry about (20 meters diameter estimate). It's possible that it would have been observed by the luck of having a telescope pointed in the right direction had it approached from the night side of the planet. Still, an object that size, it's unlikely that they'd have had more than a few hours warming.
The larger the object, the easier it is to see and once an object is observed for some time and it's trajectory has been studied, it's path can be predicted years, even decades ahead of time. The warning time drops sharply for an object not yet observed.
Potentially Hazardous Objects are at least 100 meters across, sometimes 150 meters is used as a minimum. 25-50 times the reflective surface of Chelyabinsk, so an object that size could be seen a fair bit further ahead of time.
Finally, Planet X doesn't exist and if it did exist, it would have cleared it's orbital path by now and it wouldn't be sending asteroids and/or comets towards Earth once every orbit and predicting an exact date (Sept 23rd) from an unobserved planet X. That's just silly.
There is stuff out there in our solar system that we haven't seen. Lots of stuff, maybe even a hundred or more unidentified dwarf planets, possibly another planet or two. (Click here for cool diagram)
The thing is, orbits tend to clear out over time and our solar-system is over 4 billion years old. Most of the stuff that had an orbit that was going to crash into the Earth did so long ago and Planet Nine, assuming it exists, has cleared out it's orbit and it rarely sends stuff flying towards Earth.
Space is also so big that Earth is, by comparison, a very tiny target, so an asteroid coming from the outer reaches of the solar system and hitting Earth is like shooting a hole in one from the space shutting. Stuff is far more likely to miss than hit the Earth.
But to answer your question on an object coming from a distant orbit, sent our way by a theoretical planet, it's hard to say when we'd see it in that scenario if it's a brand new observation and it would also depend on whether it came towards the Earth on the sun side or the night side. (Objects can hit the Earth from any direction), but sun side observations are the hardest to see.
The good news is, high eccentricity impacts of objects large enough to be considered potentially hazardous are rare enough that you are unlikely to be affected by an Asteroid Impact in your lifetime. It's somewhere around 140 on the list of things you should worry about. And low eccentricity orbits that could hit Earth, or NEOs, those are being carefully tracked.
The other good news is that Planet X (if it even exists) doesn't threaten Earth. No object in distant orbit around our sun would be a threat. In fact, having a planet out there would actually send more asteroids away than it would cause to fly in towards the inner solar system. They would reduce, not increase the number of impacts, similar to Jupiter.
But a large object passing through might post an asteroid/comet impact threat. Gilese 710 could do that, but it won't pass close enough to have that effect for about 1.36 million years.
I don't want to make light of this. Asteroid impacts do happen and Earth may have been hit by a very destructive asteroid or comet about 12,900 years ago (the Younger Dryas impact hypothesis) and a global changing event like that might happen every, 50,000 or 100,000 years or so, which means it's 100%, absolutely worth looking out and seeing as much as possible, because Earth will get hit by a large object again. But such impacts are very rare on a human lifetime scale.
But I'm not sure I ever answered your question. How much warning for a large object, say 150-250 meters? If the trajectory makes it tough to see, maybe only a few hours. If it approaches from the night side, maybe a few days or a few weeks. Longer if someone gets lucky and has a big telescope pointed in the right place, but I'm kinda guessing. As our technology improves, the warning period will increase. (I invite correction, especially on this last paragraph).