Watch: Zoe Saldana's Gamora Makeup Applied in Quick Timelapse Video
At the Guardians of the Galaxy Vol. 2 press junket, Zoe Saldana mentioned that the makeup process was her least favorite part of making the movie. That's because every day she filmed, she had to sit in a chair for three hours while the makeup artists worked their magic.
That's a long time, but it's nowhere near approaching the record for longest makeup application time on a single actor. I can't confirm this, but I've heard Rod Steiger holds that record for a 1969 film called The Illustrated Man, which reportedly took 20 hours to apply the detailed makeup to his body.