Size matters less than you would think
Floor area is the number people expect to be quoted on, but it is rarely the biggest factor. A large, square, sound bedroom with white walls going back to white is quick work. A small room with damaged walls, a dark colour change and detailed Victorian trims can take twice as long.
What actually takes the time is preparation and cutting in, and neither scales neatly with floor area.
Condition of the walls
This is usually the single largest variable. A wall that needs filling, sanding and spot-priming before a brush touches it can add half a day to a room, and that work is invisible in the final result — which is exactly why it gets cut by anyone quoting to win on price.
If you have cracks, previous repairs or damage, mention them when you ask for a quote. A price given without knowing about them will change once work starts.
Colour change
Same colour over same colour is the cheapest job there is. A light change is usually two coats. A dramatic change — dark over light, or a pale colour over a deep one — regularly needs three, and a strong red or deep blue can need more.
If a quote for a dramatic colour change assumes two coats, it is quoting on a finish you will not accept.
Ceilings and trims
Walls only is the base job. Adding the ceiling is a meaningful step up in time, because ceilings are slower and messier to cut in and need full protection below.
Trims — skirtings, architraves, door frames and the door itself — are slow, detailed brushwork. A room with all its trims done costs considerably more than walls alone, and usually looks far better for it.
How to get a price that holds
Send photos. For a single room, three or four photos showing the walls, any damage, the trims and the ceiling will let a painter give you a realistic figure without a site visit — which is faster for you and cheaper for them.
Say what you want included and what the finished result should be. "The walls, and I want to go from cream to a mid-grey" gets a much more reliable price than "how much to paint a bedroom".