I did mine at 40,000km when my car was 5yr old, just for peace of mind.
Because of delayed shipping, I used genuine kit, so Im selling full Gate racing t-belt kit with Grimspeed pully, all for $350, if ur interested:
http://www.perth-wrx.com/vb/sale/533...eza-parts.html
BTW, Evo R charges around $400 for the labor.