I need to know when ink cartridge in HP 9015 is too low

[SWA5]

I need to know when ink cartridge in HP 9015 is too low.  I had Epsons.  They told me to change the ink at least a month before I had to.  When I did that here, I had to have work done on the printer because, I was told, the HP gets damaged when the ink level is too low.  What is too low?  The black looks fine right now to me, but it's at about 25% and is that too low?

[Valsimot]

The printer will stop working when ink gets low to prevent the printhead from damage.

The ink level is an estimate so you can prepare for ink replacement, the printer will inform you that ink is low and needs to be replaced, you only need to have a spare ink beside.

[SWA5]

So for current HP products, when the machine says "ink low", the ink is already too low, even though the symbol says 25% of the ink is still there?

[Valsimot]

The ink level symbol is just an estimate it's not 100% correct the ink is low and the printer informs the customer that ink is low. You can get ink level symbol to get from 25% to 0% after few pages printed, and you don't want because the printer will stop printing when it reaches 0%. The printer informs user earlier to prepare ( not to replace) a new ink cartridge.

It's better explained in the user manual.

[SWA5]

I have found and read that part of the Users Manual, it only says to change it when prompted to do so.  I get told "ink level low" every time I start, is that the prompt?  BTW, I have put this message in 3x and it gets an error message and doesn't send it.