It is generally well-known that laser printers give "replace toner" messages way before the toner cartridges are really empty. There are some ways to get around this, depending on the manufacturer. Some manufacturers have a replace toner override setting but they don't highlight this feature so you need to check the user guide to see if the feature exists and how to activate it. If there is no override setting you can check online forums to see what has worked for other owners of the same model. Sometimes all that is needed is a piece of tape over a sensor.
No matter how you override the toner warning just keep monitoring the print quality to see when it's really getting low. When the image starts to fade during a print taking the cartridge out and rotating it a bit usually will give it enough of a boost to improve the output. When this trick doesn't work any more then you really know that it's time to replace the cartridge.