How Long Does It Take For A Cat To Get Used To A New Kitten?

My cat is 11 years old, she never got used to my new kitten at all. I’ve had her since she was 8 weeks old and now he’s 3 months old. It took about 2 weeks for them to get together but they’re both doing great now! They play with each other everyday.

How long does it take for a cat to learn how to open doors?