On PlayStation EU Blog site, there is a Q&A about Killzone Shadow Fall, and one of the things that was asked about was how long is the campaign.
When asked "How long is the campaign?"
Game Director Steven Ter Heide, did answer with
"Killzone Shadow Fall’s campaign should last well over 10 hours for most gamers."
There are many other questions asked that now have answers to on the PlayStation EU Blog site.