The books are still being written, and the show is following the plots written (for the most part, although there have been some changes). I dont see why they would stop before they reach the end unless at some point the show stops turning a profit.
How much longer you think it's going to last? Just because the comic is still going doesn't mean the show will keep going. The longer the show, the more it costs them to run and harder to turn a profit.
I have no idea how much longer it will go.
The only thing i know for sure is that if the series ends before kirkman finishes the books the tv show cant use his ending.
As of now in the books Negan is still alive and so are Rick, Carl, Maggie and Michonne from the main group that exist on screen and the page. Carole on the show seems to be headed towards getting Michonne's book role related to the Kingdom. (Shes been dead in the books for ages.)
