2-3 generations, maybe more. Depends on the person really. Some are fine still running a Pentium 4 PGA478, some went from a LGA1366 to LGA2011 right away. Just depends on what you are comfortable with.
With each release of DX. 8800GTX was still doing okay in games when DX11 released. GTX480 will still be doing just fine when DX12 is released. If you game heavy, you might want to upgrade every other generation though. HD3k, HD5k, HD7k and the like.
When your RAM starts failing, or when you switch motherboards. DDR2 is DDR2, just as DDR3 is DDR3. No real need to replace, just add more.
About the same as motherboard. Only upgrade when you feel the need. Some still rool with Pentium III/IV, some feel they need the newest with the i7-3960x. All about the user, and their needs. Most upgrade before they need to.
when you run out of space. No real need for upgrades there at all, except to larger drivers.
every 5 years or so. If you have a crap one to start with.
When it stops working, or when a newer really better one comes out. Again, dependent on the user.
when they break, or when the interface changes.
when they break. No sooner really.
every 5 years. Some will last longer, some will not. But this is about the life time before you replace them.
when you feel the need. Case is personal preference.
Extra:
Screen
Mouse
Keyboard
Speakers
Dave
these are all personal preference. no real need to upgrade them unless you feel your current ones are not working to your content.