The usual figures given for PWRs are something like "These reactors have the capability to regularly vary their output between 30–100% of rated power, to maneuver power up or down by 2–5%/minute" (https://en.wikipedia.org/wiki/Load_following_power_plant#Nuc...). This means that you can regulate your PWR over its power range within slightly more than half an hour at worst.
There is a lower limit to how much you can regulate your reactor down due to various limitations like Xenon poisoning. Because of that, a controlled complete shutdown is slower, because you need to get through the "icky" region between e.g. 30% and 0%. Also, you usually prepare for the longer phase the reactor will stay shut down. But all in all, shutdown is different from load following, and a conclusion that nuclear plants can't load-follow from a question about shutdown is just asking the wrong questions and drawing questionable conclusions from it.
While you can do an emergency shut-down quickly, normally adapting the power output of a nuclear plant takes many hours:
https://www.quora.com/How-long-does-it-take-to-shut-down-a-n...