My study plan always looks better on paper.

I can make a really detailed plan and everything seems completely manageable. Then I actually start studying and one chapter takes twice as long as I thought it would. After that the timings are off, I start moving things around, and by the end of the day I am basically ignoring the original plan anyway. I've started leaving more empty space in my schedule because apparently I have no idea how long anything takes. What do you guys do when one task takes way longer than expected and throws off the rest of your study plan?