I've got the Costing formula part way down the page listed below, but I don't recall seeing anything about the time formula
http://www.icewebring.com/rmcr/characters.php
From memory:
Base time is 2 months per category/skill rank
+50% of base time if TP is lifestyle
+50% of base time if TP gives stat gains
+50% of base time if TP gives spell list access
+/- random number at Editor's discretion.
Best wishes,
Nicholas