The length of your cat's hair
Your cat's ability to groom herself
Cats who are overweight or ill often have less ability or desire to groom themselves. Older cats who may have arthritis may also tend to groom less. These cats should be groomed on a more frequent basis.
The amount your cat is shedding
The amount your cat sheds varies throughout the year. When your cat is shedding less, especially if she's a "shorthair," you may brush or comb your cat and remove very little hair. Other times, it may seem to come out in handfuls. The more a cat is shedding, the more often you should groom her. This will also greatly decrease her risk of developing hairballs.
Your cat's tolerance to grooming
Some cats love to be groomed - giving you excellent "bonding time" and a beautiful cat. Conversely, some cats seem to despise brushes and combs, and should therefore be groomed only on an "as needed" basis.