Showrunner Steven Moffat: “I don’t know how long we can keep it going. I’m personally willing but I’m hardly the main draw. I would be moderately surprised if this was the last time we ever made this show. But it absolutely could be.”
“There’s never going to come a time when we do a longer run, because this is what the series has become. It’s an occasional treat where you get three movies. It’s how it works.”
Moffat said that if the pair’s schedule became too hectic to commit to future series, the corporation could return to the show years down the line.
Source/more: The Telegraph