I have a long haired foster, she was medium haired when I got her at around 1.5 y/o, alot like your black kitty. I have had her 3 months and when the cold weather came in, her coat poofed out big time. It's thicker and longer too, she is definitely what I would consider long haired now. I would bet yours will get fluffier still.
Muffin, Zoey, and Cheddar Biscuit