Depends on the astro and how much grip it has. Many tracks in the UK are very high grip and you can rinse a set of rears in 2 runs. The top guys often only run once in a National but mortals can run a bit more.
We mostly use Schumacher mini spikes or Darts on astro. The Darts in particular give great initial grip but fall off badly. Mini spikes are a bit better but a competitive meeting would usually be 2 sets of rears for me ( 4 qual and one final).