Time has been passing. And I’ve studiously ignored the warnings from the printer, and kept on sending it jobs. Which it has kept on printing. Beautifully.
I’m now at a total of 885 impressions (348 mono and 537 colour) and despite complaints that the magenta cartridge is now also “low”, there seems to be no difference in the quality of the output. I’ve now got replacement black and magenta toner cartridges “waiting to go”, but see no reason to install them until the print quality starts to actually degrade.
I’m cynically starting to wonder if Lexmark have set the warning levels artificially early as a means of promoting toner sales. It will be interesting to see if the printer enforces replacement before the toner actually runs out, and how much toner is left in the cartridge at that point…
Richard's Blog 
I experience the same thing on my Epson. The escputil (tool for reading Epson ink levels on Linux) reports 10% remaining when the light starts to flash. So as soon as one cartridge gets to 10% then I get ink low warnings. Rubbish!