It varies is the short answer. Some of the older track circuits, which can be over a mile long, are actually broken into several smaller track circuits and indicated as one. Radio frequency track circuits and axle counters do away with a lot of that, because they aren't affected by current resistance in the rails.
Track circuits are usually provided where:
There are points, signalling overlaps, berth track circuits (on approach to signals), clearing points, between signals, across ground frames and in association with signalling controls, such as approach control, calling on, warner routes, swinging overlaps and so on.
Signal spacing is dependent on line speed and gradient, but take a main line is typical:
2 aspect: Varies as stop signals can be many miles apart (usually at least 2-3). Example lines: Norwich (Trowse)-Stowmarket, Banbury-Oxford
3 aspect: 1-1.3 miles apart. Example lines: GWML-Reading-Didcot, ECML Newcastle-Berwick, Parts of MML.
4 aspect: around 3/4 miles apart: Example lines WCML, GWML-Reading, ECML (except slow lines and Newcastle-Tweedmouth).
