Unless your really really fast and carefull, its nigh near impossible to do the whole thing in one go around, because the resin hardens so quickly. Another plus to doing it in sections is that, since everything on wont be wet, there will be more structural integrity to support the wet sections.
Im doing mine in chunks. I did the top middle section yesterday and then started working on my left thigh piece while that dried.
If you have the right mixture of hardener and resin, ideally, it should dry in about 20 minutes.
Personally I use 25 drops of hardener per 2 OZ of resin. And I usually have about 7-9 minutes to spread it around before it begins to goop up.
Point being, you can easily resin an entire helmet in just a few hours if you want to, but dont rush it, because it will turn out badly. My first helmet was sorta lopsided cause my dad and I did the entire outside in 10 minutes, so yeah....
Fiberglassing will take a bit longer, just cause you have to cut and fit, and such, but I guess, you could have a helmet resined and fiberglassed in one day. I cant, cause I have work from 8-5 =/