If you count being forced in a box by government entities as imprisoned, he's been imprisoned since 2012. If you're a stickler for being literal, he's if nothing else been captive since 2012.
Not to mention the UN's guy whose job is assessing whether a person is being tortured has repeatedly said that yeah, what's being done to him counts as torture.
«Painting a picture of progressively severe suffering inflicted on Mr. Assange from his prolonged solitary confinement, the Special Rapporteur upheld that it not only amounts to arbitrary detention, but also to torture and other cruel, inhuman or degrading treatment or punishment.»