How long will my cats hair be and how do I groom her?

She will have long hair (she already has long hair), it will be longer when she is fully grown than it is now.

The best types of brushes I've used for long haired cats are these. There are many other types of brushes available, but these are the ones I've preferred over the years. You basically need a brush with divided bristles, you can also use metal combs.

They are good at getting out shedding hair and untangling basic knots without hurting the cat. There will be places, for example behind the ears, that are prone to becoming matted.

As mentioned by Trond Hansen in the comments:

Try to brush the places your cat can't reach when it washes itself, right behind the front legs and lower back and under the chin chest and around the head/neck area

It's important to stay on top of these. Checking around the neck, under the legs (right in the armpit/legpit area) and behind the ears regularly. If a small mat appears, it needs to be cut out with a small pair of scissors, with care taken not to cut the cat's skin. Gently pull the mat away from the skin and cut at the base of the mat near the skin. Some of the mat may remain at the base near the skin, but it will generally grow out as you brush the hair.

It's best to start a grooming routine now. A daily brush will accustom her to being brushed and assist in preventing mats. By getting her used to checking under her arms and legs and brushing there, it will make it safer to handle her if you ever do need to use scissors to cut out any mats.

Matted hair is common with long haired cats. There's no failure in finding small mats in your cat's coat. It's important they are dealt with quickly. As the matted fur increases, it pulls on the skin and can become painful. Large areas of matted fur is commonly seen in long haired pets that are neglected and can be construed as a form of animal cruelty.

So a long haired cat is a responsibility, but also a joy, as the hands on grooming assist in developing a relationship with your pet.