CR2032 coin cell 3V battery @ what vcc level before changing for new

[bigsanta]

CR2032 coin cell 3V Lithium battery and i wondered at what discharge down to voltage level would it be considered to be time to change it for a new one ?I wondered about this because of a low level battery indicator circuit i want to build(led would pulse at a very low freq to show the low battery level and a very low duty cycle)  .All the ones i've come across so far,just don't work with such a low level of voltage either.