When to cluster a table using an index? [postgresql]
If the index that you use for clustering is on column col of table public.tab, run
SELECT correlation
FROM pg_stats
WHERE schemaname = 'public' AND tablename = 'tab'
  AND attname = 'col';

If that value is 1.0, the order is perfect, if it is close to 0, the order has rotted. You best use trial and error to find from what value of correlation on performance gets bad.