How long does/should it take a decorator to line 2 walls before painting?

Should get the lining done in the day and pain when all is dry and ready to go, so the next day I’d say And if it is a flat wall a then you should be doing it length ways

2 walls lined should take no more than 4 hrs and should always be crossed lined so there is minimum joints

1200mm??? What lining paper is that?? Do you mean 1200 grade? That relates to the thickness of the paper not it's linear dimensions. Is it non-woven lining paper? Is there any prep to be done? Is the room clear of furniture? Do the walls need sizing first? As to whether it's hung vertically or horizontally. Whatever works out easiest

All depends on the substrate!! If there’s not much preparation needing done ? Then a day should be more than enough.