Well, today I installed a black (BCi3e), cyan & magenta cartridge from Premier Ink Supplies (£9.93in total, in. delivery).
As a "control" I used a genuine Canon yellow cartridge (£6.99).
The yellow is suppoed to run out before the others, so if this proves not to be the case, then a lesson can be learnt!
Unfortunately, there appears to be no way of printing a configuration / status page*, as on my laserjet, that shows the number of pages printed to date, so I won't be able to calculate the number of pages printed between replacing cartridges.
* - if anyone finds otherwise, please let me know!
I'll keep you posted, but don't hold your breath, as I don't do a lot of colour printing!
SD