I had more then third of adblue in tank (larger volume 24l) when left on holidays, expected Route plan up to 3k km. The warning appeared shortly after 500km when 1/3 tank indicated. I ignored the warning untill time when general warning for refill became more precise saying that system will switch off after 756km. Roughly: first general warning came 1500km before limit when engine wouldn’t start no more.
Refill process: adblue tank was at 1/4 (6l) nothing has changed after refill of 5l can, the warning disappeared after second 5l can.
It was my first adblue refill in MP, according to my calc: 1l adblue for 600km / engine 250D, 4MATIC