Update roughly as follows
Case: As required (usually every 5 years or so)
PSU: As requested (rarely more than once every 4 years)
Motherboard, RAM, CPU = Every 3 to 4 years.
HDD - Usually one change a year (I have 8 HDDs).
Graphics card - Every 2 to 3 years.
Peripherals - Every 2 to 3 years, although finding Logitech stuff is lasting longer now.
Monitor - Every 5-6 years as technology changes and cheapens.
I but most of my equipment second hand, in my current setup only the monitor (HDTV) and motherboard ( Asus P8Z77-WS ) and my keyboard is bought new, everything else including all 8 HDDs are second hand. This drives the cost down and I tend to upgrade using OCUK MM as the guys who like to be at the cutting edge are shedding their year old gear. I then resell my old PC to someone, never had a problem shifting it in the past.