There is no actual 'sure' life to a cartridge, because it all depends on usage, printer set-up and capacities.
I use a HP Photosmart C3180, which now holds and uses higher capacity compatible cartridges, which in turn makes a great financial saving. Original HP 338 black 11ml (compat 20ml) HP 343 colour 7ml (compat 14ml).
Regarding the warning messages, I use the printer until a poor quality print turns out, usually many many copies after the warning. HP printer are not alone on this procedure. Other brands seem to have similar early warning devices.Not very eco-friendly perhaps!.