Over here, the thickness of the finish coat depends on the type of plaster, and what the finish plaster is going onto.
For finishing over drywall (plasterboard) and probably most surfaces, I like to see a minimum of 3-4mm because it will cover the scrim joints, and give good coverage on the angle beads.
Finish on top of sand and cement render, or bonding coat may be a little thicker still by time it has filled any keying lines etc. Although I’ve often seen it very thin.
When skimming over old finish, we normally first coat with Gyp-bond or similar which is textured PVA. Then skim finish, if no angle beads are used, I’ve seen the finish as thin as 1mm, which often cracks.
All said and done, providing angle beads are covered, and scrim joint etc, and the overall is flat, the finish can be various thickness.
Generally though, it will be 2-4mm
I favour a good thickness to help eliminate cracking.
That really goes for other types of plaster too, for bonding plaster, on very uneven walls, the thickness before the finish goes on, could be between 3-25mm or more.
The bonding is applied to pull out dips and uneven surfaces. Quite often this type of plaster will be around 13mm before finish.
I’ve taken out brick walls where the bonding coat has been 70mm in places. In that instance personally, rather than load the wall up with thick bonding, I’d dot and dab the wall nice and plumb, then skim it.