The legislation requires the “Driver card” (digi card) to hold a minimum of 28 days data. This is based on 98 (not sure of correct number) activities per day.
The data is over written when the chip is full.
This means that I can go back twelve months on my card as I don’t drive every day.
Just scroll back on the the date when doing a print out, to see how many days are stored on your Driver Card.