I haven't changed my hairstyle in quite a long time! When I first unlocked Shampoodle, I changed it almost every day, but it was only to unlock the girl hairstyles. Nowadays, I keep it brown and I guess medium length to short.
I really like the girl's hairstyle where you put the slider all the way down, say "private", and then say "it's a party!". I really like it where you have the color as "the color of burning love". I've just recently changed it to a brownish color, but that was just to make it look a little more like me. I really prefer the red, though.
As for how often I change it? Like, every couple days. Except for when I changed it to be that brownish color; I've been too lazy to change it since then.
I've never really changed my hairstyle in any Animal Crossing game. I make it look like my hair, then just leave it. I never take breaks long enough to give my residents bedhead, so there's no need. p n p