The Evil Within's Terror is going to last you between 15 and 20 hours according to Bethesda Europe's PR manager Alistair Hatch.
When asked about the game's length on Twitter, Hatch said, "Saw some people asking about it - we expect The Evil Within campaign to last 15-20 hours depending on play style and difficulty mode."
This is the first time the game's length has been answered with an approximate time by hours. This time also only includes the on-disc campaign and does not include the 3 additional add-on DLC's coming later next year.
The Evil Within releases on October 14 and 17th in America and Europe respectively.