Depends on how accurate you want that pack to be, really. In an afternoon using cardboard and found objects you can put together something that looks close, something that people will see and say “Oh, Ghostbusters!” at a party. But if you want something detailed, you'll need time, patience, and some skills–but if you give yourself enough time and have enough patience, you can acquire the skills as you go.
Takes most people about three months to put together a high-quality pack, but I remember that someone put a great pack together in a weekend because they literally did nothing else–the only thing that slowed them down was waiting for paint and glue to dry and stuff. But if you have a life–school, work, both–then it's good to leave yourself at least one month.