I've just had to replace the right head in my iPF8300 for the second time in 18 months, following an earlier service at which both were replaced by the technician (possibly unnecessarily - the root cause of the problem at that time may well have been a faulty data cable, see thread elsewhere). The first replacement right head failed at around six months, this one at a further 350 days or so, both with the same issue of 'echoes' in the nozzle check - i.e. a second set of fainter vertical lines offset from the main lines in one or more channels.
There was no obvious visible degradation in print quality, and the printer's routine nozzle checks did not report any errors, but some profiles I attempted to make last week were full of 'wormholes', inversions and jagged spikes, which is what first made me run a manual nozzle check.
The left head is now 556 days old and has a similar if less visible issue in the yellow channel. I also found a previous nozzle check (date unknown) which on close inspection showed a similar echo in this channel that I had previously missed, which leads me to think this head has also had undetected problems for some while.
The head usage figures are 91 Mdot for the right head and 189 Mdot for the left; the UK suppliers have agreed to replace the right head under warranty (it was fortunate that I found the problem just before the 365 days were up), but won't do anything about the left head as it is now over a year old, despite the low usage figure.
The printer is left on to do its own routine maintenance, is used regularly if not heavily, is kept very clean and is protected with a dust cover when not in use, etc etc. I am beginning to have serious doubts as to the longevity of these heads, and Canon or their agents in the UK do not appear disposed to offer any concessions in respect of early failure, contrary to reports I have read of their more helpful attitude elsewhere.
The lesson seems to be to keep track of the head age and to check very carefully in sufficient time before the year is up to claim a warranty replacement if necessary.