I've heard that the Shampoodle opens after you spend 10,000 bells at Kicks or the Able Sisters. Does it start building the day after Kicks, or another random day?
From the Animal Crossing Wiki :
It must first be unlocked by spending at least 10,000 Bells between the Able Sisters store or Kicks. Kicks must also be open for at least 7 days before construction begins. It takes 4 days for Shampoodle to be built.