Calculate the room volume! It's not impossible to fill any room with a lethal amount of CO but the smaller the room is the better it is. I'd say a conservatory may not be as airtight as it looks like. For the time CO needs to reach other rooms it's impossible to give an estimate here bc many factors play a role here, e.g. doors, thickness of walls, material and so on.
How do u plan to generate the CO?