The day he retires/chooses to leave, it'll be because he's mentally done with the game, rather than physically. 16 years in the Premier League? Crazy.
He's been managed well and retired from internationals, so I reckon he could physically do another 3-5 seasons yet, making atleast 25-40 appearances every year. He's made 36 and 32 league appearances for us in the last two years and he's looked the fittest during games. It's whether he can be arsed mentally, and under Klopp I think there's no chance he packs it in, knowing he can win another title.