The depth of the soundstage is determined by your equipment, venue, and recording. Assuming a good recording that can have depth… many. I don’t want to get off onto the details of this subject.
My system images well (see mine under my UserID). When I finished up positioning the speakers the soundstage would extend two or three feet beyond the speakers to the side and back about 4.5’ to the wall. Then I hung a very heavy tightly woven wool carpet on the wall… now the soundstage goes back into the wall two to four feet more depending on the recording.
In previous systems, initially the soundstage only extended to the wall. Then, upon careful tweaking the wall ceases to truncate the soundstage depth.