A piece the size of a fingernail, ten minutes, and a window open. Most people use three times that and live with it for two days.
Bakhoor instructions are written for large houses with high ceilings and plenty of air moving through them. Most of us live in flats with sealed windows and air conditioning, and the same amount in that room is overwhelming.
The quantity that works in a flat is about the size of a fingernail. One small piece, on a low heat, for roughly ten minutes, with a window open a crack or the door to the stairwell ajar. That is enough to scent two rooms pleasantly and to have faded by the evening.
What happens when you use more is not simply that it smells stronger. Smoke settles into soft furnishings, curtains and clothes hanging in an open wardrobe, and it stays there for days, getting staler as it goes. The lovely smell on the first evening is the sour smell on the third.
On burners: charcoal gives a better result and needs attention, an electric burner is duller and is the right choice in a flat with small children or where somebody has asthma. We will tell you that even though the charcoal burners look better on the shelf and cost more.
Timing matters as much as quantity. Burn before guests arrive, not while they are sitting there. A proportion of people get a headache from smoke in a closed room and almost none of them will say so in your house.
And a note for anybody scenting a guesthouse: do not burn bakhoor in bedrooms at all. A guest who dislikes it cannot leave, cannot air the room properly with the air conditioning on, and will write about it. Scent the public areas lightly and leave the rooms clean and neutral, which is what most people actually want from a room they are sleeping in.
