Epson L210: It is nearly time to reset the ink levels 

Don't panic!
When a window pops up on your screen saying: "It is nearly time to reset the ink levels." the printer keeps working.
The "Ink drop" lamp under the right hand warning button will be flashing. It will stay flashing and the printer will keep working for several more prints.
But you must check the ink levels and fill the bottles to the top if necessary.
Eventually, after several more pages have been printed, a different window will pop up saying:
"It is time to reset the ink levels".
Then the printer won't print and the "Ink drop" lamp under the right hand warning button will be on permanently.
What you must do is hold the right hand warning button in for about 20 seconds and then release it. The lamp will still be on. Press it again for a second and the orange "Ink drop" lamp will go out and the printer will print again.
This is the second time I've done this with my L210, so I was ready to video the process and upload to RU-vid as most of the other videos on the subject are not too clear.
I'm not sure about the "20 seconds". It may be sufficient to hold the button in for just 2 or 3, but I thought I'd make sure.
UPDATE JUNE 2019
I got the "It is nearly time to reset the ink levels" message again a few days ago.
I tried the procedure above but, as usual, the "ink drop" lamp stayed flashing.
But today, after I got the message and printed a page, I did the procedure again and it worked - the flashing "ink drop" lamp went off and the next page printed without any message being displayed.
So it is probably worth trying the procedure after receiving a few of the "It is nearly time to reset the ink levels" messages.
You may be lucky to get it reset like I was.