Updating El Capitan to High Seirra will take 18 hours
None whatsoever. 

Leave it alone, it will get there.

If it's claiming such astronomical time remaining, it's based on 'how fast it went so far vs how long it thought it should have taken'. That's a variable dependant on many things - HD vs SSD, space remaining on drive etc.

Also, your download speed will have an effect - maybe significant if your connection is slow or intermittent. [I've never had a download be slow or fail, but I would assume the installer is capable of handling a failed download gracefully].  

It will recalculate periodically as it works, so you might find it will drop to something more sensible as it progresses.

One thing to watch out for… when it gets to "1 minute remaining" don't run out of patience at that point. It is really saying "I've done everything I can measure &amp; these final tweaks should only take a minute." If it has a lot of files to look at, it may take hours. It can't tell you at that point.