Hei!

I'm new to shooting day to night with a slider, so I'm not sure how it's done. 
Since the exposure time will vary, I will need to set the slider for the longest exposure time I will need during the shot? In other words, I need to guess my longest exposure time during the sequence?

Normal you set the interval to a fixed value (longer then max exposure). The Slider will be in sync with the interval, because in the most serious the slider controller will trigger the camera.

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Normal you set the interval to a fixed value (longer then max exposure). The Slider will be in sync with the interval, because in the most serious the slider controller will trigger the camera.

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And the fixed value, how to I determine the interval? Because I set up my slider during the day, and I don't know how long my exposure will be when it's getting darker. If I set the interval to 10 seconds, the exposure can be no longer than 8 seconds, or perhaps 7 seconds. If I need more than 7 seconds, do I need to up the ISO? I suppose I can't change the interval on the slider once I start it? And if I do, the movement will only go faster and I don't want that.

Yes, you'd continue with the ISO once you reached the longest exposure (Interval-Darktime).
Check out my holy grail tutorial:
https://lrtimelapse.com/gear/dslrdashboard/
It might be a good idea as well to get yourself a copy of my Ebook "Timelapse Shooting and Processing" in order to learn the basics: https://lrtimelapse.com/lrtimelapse-pro-timer-free/