How much time would it take to reattach a zip slider to one side of a jacket inside pocket?

Justv the slider? A couple minutes tops

I depends on why it came off in the first place.  Did it come off one end of the zip, or did it come off in the middle ?  If the latter, it would be difficult to re-attach without taking out one end of the zip - and if it came off the middle it suggests some damage to the chain, and the slider would always be liable to come off again.  It might be better to see if you could get the whole thing replaced.

Better to have a reason to keep it open. Like where you keep a sandwich, or a flask, or your emotional support ferret.

You can't easily do that without undoing one of the ends of the zipper. By the time you have done that it would be easier to replace the zip with one that won't break. Without seeing it hard to say, But I would probably undo some of the lining to get at it. Could be about an hour or so if not more - most zips are inserted early in the construction process of the garment.