The impression I was left with as a viewer was that it was probably like a few months-year? They don’t elaborate much about how long the trip to Saturn or the planet hopping took. But, if we assume that the endurance had some near future techie stuff like NERVA motors (plausible as the endurance didn’t have radiators or solar panels) and you sprinkle in some DV for the sake of narrative, probably significantly faster than any manned craft we have today, but certainly not relativistic. Then again, since the movie is about the relatively different experience of time for Murph and Cooper, it’s ultimately not very relevant to the story.