Nobody knows for sure. Most probably it would have dragged on for much longer than it did. In my opinion france would have surrendered under some kind of truce when their soldier's refused to fight in 1917, and britain would have become crippled under the constant you boat attacks from the germans but I don't think they would have been conquered completely. So all in all, it would have dragged on for years longer and germany would have conquered much of europe and western asia.