Those parallel grooves are the manufacturer's markers. When they are gone is when they should no longer be used.

And I would changing the discs aswell looking at them pads

i'd change with 3mm left only due to the extra travel the pistons have to move unless self adjusting which i assume your Duke hasn't got, nor do i know of any bike that does, too easy to replace them so its a pointless advantage.

Richie848evo wrote:And I would changing the discs aswell looking at them pads

Not sure how you can make that leap.
Yes there is some marking in the pads, but that is due to me wedging the pistons back in with the old pads in, before installing the new.
I will have complete confidence in the discs piling up to park bend "200" board at 130mph

Cabernet wrote: I will have complete confidence in the discs piling up to park bend "200" board at 30mph

Fixed it for ya!