Took me 6 months to complete my Sig Cap, working a wee bit at a time. Took me one evening to get my ugly stick in the air. Just depends on how long you go at it at a time, and how easy it goes together for you. What glues you use, your building abilities, attention to details, trips to the hobby store for the forgotten extras, it all adds up. Some can get it done faster than the box says, some take much longer.
It's not like everyone will complete it in exactly 6hrs, or 8hrs.