See "How big Moodle really is (code-wise)" https://moodle.org/mod/forum/discuss.php?d=244442. Warning: From year 2013!
Thanx Tim an Visvanath ! Both your answers confirm that it is in the range of several hundred person years. That is good enough FYI I am writing a manual for a distance teaching university and just wanted to have some evidence that creating some good and useful software takes a lot of time and person years (and the result is not even guaranteed as more recent attempts at creating second generation LMS show .....)