I'm sanding the glossy paint of my bathroom before painting over it. But I realize it's hard work and I find the sanded surface still kinda glossy. Actually I have no idea how it should look like in order to qualify as a suitable surface to paint over it, nobody discusses this aspect in the web pages I came across, they just advise to sand. But to which extent, until seeing what?
Surface prep is one of the most important factors of painting, it takes most of the time. If the surfaces are clean of environmental contaminates, fireplace and cigarette smoke for example or hand prints around door knobs, then you could forego wiping down the trim or places were heavy deposits of "stuff" build up.
There are "liquid sanding" compounds that dull a surface to eliminate sanding, But I can't vouch for those. Hand sanding at best for detail areas, use a pole sander for larger wall surfaces, use 120-180 grit if you plan on priming the wall again. This will be needed if you plan on changing the color to something that may not cover on one coat. Use 220 to de-gloss the surface, if the finish paint can be done in one coat. Otherwise you can see the scratch marks through the finish coat (with heavier grit), if you use gloss paint. This effort over the old paint will also remove other debris that inevitably gets in the wet paint as it is applied or while it dries, such as lint, hair and fuzz, paint runs, you name it.
Use the what may be now the not-so-reflectiveness of the once was glossy paint to determine how well the scratch pattern is on the wall.
With a pole sander, it may only take an overlapping pass in each direction (left to right, top to bottom) to accomplish this. This will help keep track of what you done already. Better than waving around all over and hoping you caught all the area.
No sanding job will be 100%, the idea is that the new paint will have enough to bite into to hold up over time.
The visual amount to look for? Picture what a comb or brush for your hair would look like if it left marks on the wall after one or two passes. Of course, what you will be doing in reality will not be uniform as my word picture suggests, but the idea is to have more scratched wall surface than glossy, with no large spots that are still glossy like it was never sanded.
Do a test. Lightly sand an area. Heavily sand another. Maybe sand another sort of between the two. Then prime and wait for it to dry. Then try to scrape it off. This will tell you more that you will learn any other way.
If the primer does not adhere well to any sanded area, you need another primer. And, if you are not using an oil based primer now, I suggest trying that next.