Yes, it is time for DD to go. He hasn't exactly got a track record of getting things done and judging by his wooly, bumbling and often incomprehensible explanations, can't exactly be mustard at the negotiating table.
Contrary to popular belief, there isn't a shortage of competent, clear-thinking and savvy Tory Brexiteers who could probably handle the negotiations much better. Theresa Villiers, Nadhim Zahawi, Michael Ellis, Dominic Raab and Adam Afriyie spring to mind. In my opinion, the longer Davis is in the driving seat, the longer the negotiations will stay on the road to nowhere.