The process of upgradeing:
Powersupply, Motherboard and Processor are upgraded every year.
Memory and graphics card are upgraded every 2 years.
Perperial devices, such as the mouse and keyboard are upgraded when I feel I need a new one, about once a year or more.
Hard drives are upgraded/added about 1 and 1/2 years.
CD-Roms, ie dvd/cd/rw/cd-rw/dvd-rw etc ... Are upgraded about 2 years.
The monitor and Case are about the only thing I have not yet upgraded and I probably won't upgrade them for about 5 year intervals.
Thats basically how I have been upgradeing over the past 5 years now, I have not had a need to change my case or monitor yet, but I feel like I might get another monitor so I can have dual monitors
(My current monitor is a sumsung 950b)