If you search for "duty cycle" you should find some info, it's very subjective as single pages are more wearing and cause reduced life (the fuser and engine are cycling more), media type can cause increased wear etc.
http://h10010.www1.hp.com/wwpc/pscmisc/vac/us/product_pdfs/473038.pdf
This is the leaflet that shows the Duty cycle for the CLJ4700/ CM4730 engine.
As far a life expectancy, I don't believe there is a formally quoted figure now days, there are often printers working wll beyond their expected life due to use of good quality media, regular maintenance and good quality consumables, whilst others fail far short of any expectation due to poor media, maintenance and consumables.
I've seen LJ4100 for example at 4.5 million pages in almost factory condition, whilst I've seen others barely at 0.5 million that are beyond salvation. As page count increases wear and tear comes into play and some parts will need replacing but that's expected anyway.