Oh, crikey. Where to begin?
If you're experienced with actually assembling the Pepakura form, you might find that you don't spend as long as you thought on that stage. I personally take perhaps two days to knock out a reasonably-detailed helmet, a chestplate could take me up to a week depending on the quality of the file I'm working on. Once you get into a groove you'll find that the basic product assembly won't take you as long as you thought it would - just practise, make sure your edges and corners all line up nice and neatly.
Following that... the 'hardening' stage might take an afternoon per piece. Getting your resin applied in nice, even, thin coats won't take long if the weather is on your side and you're working at a measured pace, and if you're working a piece at a time, you should be able to perhaps get a layer or two of fibreglass on your piece as well before it's time to pack up and head back inside. I certainly recall a few instances over some weekends where I could get an assembled helmet hardened (resin
and fibreglass) in an afternoon, if I got outside in the late morning and packed up mid-afternoon or when the temperature began to drop.
The last stage - smoothing - is The Big One. This will take you as long as it takes, honestly. This is dependent upon how large the piece is and how much attention to detail you're putting in. If you've nothing on your hands but time, and you're working on a piece full-time, expect perhaps a week or two from applying your filler to getting it finished and ready for paint - though, if you're working on this project as a hobby, this will obviously take a considerably longer period of time to finish up. The important thing to remember here is to
be patient - smoothing takes a
lot of work, but it's
definitely worth every bit of the work you put in when you see the results at the other end.
And, most importantly of all:
don't rush! Any part of the project you rush on risks producing sub-par results - as
Dirtdives said, it's not a race. Don't impose deadlines on yourself, work calmly and don't force yourself if you're not in the mood to do anything.
If you're looking for a 'quick and easy' build you could throw together on a shoestring, I might suggest looking into foam instead.