I've heard around 4000 a year. But these two links support your 3600 per year belief (300 per month)

http://cars.about.com/cs/testdrives/fr/wrx_sti_tst.htm

http://www.thecarconnection.com/Vehi...184.A5992.html

However Soa themselves said 7200 for my (model year) 2004 according to this post:
http://forums.nasioc.com/forums/arch.../t-607257.html