A few hours. The Catwoman one was sewn, except for the ears which took some light shaping. The Batgirl one was built up with tape and cardstock. The important thing is to try it on regularly, don't get too far ahead unless you know it fits right or else you'll have to backtrack a lot.
I work on these over many days when I have a few hours of spare time, but they come together very quickly and start looking like what they're supposed to be.