Whenever I spot a 'bargain' that will give me at least a modest increase (say 30%+) over what I currently have.
That said, what typically happens is something along these lines:
Case / PSU: When I have to, current case is over 11 years old
Peripherals: Usually to keep in line with technology changes e.g. TFT, 120hz monitors, mech keyboards, optical mice etc. Say every 3 years on average perhaps (I used to change mice pretty much every year, but not so much now).
Mobo+CPU: Tends to work out as being roughly every 2-3 years. I always change both together.
RAM: Alongside the above but often with an additional upgrade inbetween as prices drop
GPU: Various depending on 'bargain' factor mentioned above but roughly every 18-24 months on average I'd say
Disk: As and when required, I'll probably get another SSD though if there is a big price crash (say 500GB+ for under £100). At the moment prices seem very static though, it still seems to be around 50p+ per GB (old/slow tech excluded) and has been for ages.