The spaces left by extracted teeth start closing as soon as the braces are fitted, but slowly. The main redistribution of space in the arch happens during the working phase of treatment. By then the archwires are stiff and thick enough to let the brackets slide along them towards each other, like on a rail, pulled by elastics or springs. The space left by extracted premolars is about 8 mm on average. The normal rate of space closure is about 1 mm a month.
This answer is for general information. Your own case needs an examination by an orthodontist.