Well, as for the depth of the image and how long it may be... First off, perspective is definitely not my strong point and I'm a noob. Second, in general, it doesn't have to be exact in art, just needs to work/be convincing (which it is, I find this drawing quite pleasing!)
Now, to be more exact, in perspective, things get a little distorted and what you are left with is relative proportions so there is no one universal measure but rather scales
One easier way to do it is to have a scale beforehand, for example making a rectangle on the floor and subdividing it using rectangles and diagonals. The other way around is making rectangular grids where you start with one rectangle, measuring some known variable (wall height, an object etc) and multiplying it in perspective to measure out the distance
hope this helps
cheers!
