During the early weeks of a litter's life, weaning is the process of a mother cat slowly but surely dissuading her kittens against full-time nursing, and gradually introducing them to the world of solid foods. Weaning, in its entirety, usually takes a few weeks to complete. The job isn't instantaneous.
Mother cats are in charge of determining when exactly to wean their litters off of exclusive milk, although the procedure generally starts when kittens are somewhere around the one-month landmark in their wee lives. When a queen is nowhere near and you're in charge of bottle-feeding, you can also start weaning kittens who are roughly 4 weeks old, too. Commercial kitten milk replacer is the way to go for those purposes.
Length of Time
From the very beginning up to the end, when a mother cat is no longer ever nursing her kittens, weaning usually lasts from a month to about six weeks total. If a litter of kittens is on the fast track to eating solids, they may be completely weaned at a young and sprightly 8 weeks old. However, it is also common for kittens to finish weaning at around 10 weeks, as well. Remember, all kittens -- and mother cats -- are different.
You may observe, occasionally, that a kitten is done with weaning but still likes to suckle. If a kitten is only eating "real" food but still has a penchant for suckling behavior, it may be an attempt to soothe and relax himself -- aww. In some cases, motherless kittens who undergo weaning prematurely develop suckling issues as adult cats, whether they obsessively put their mouths on your hand, on your bed sheets or anything else.
When kittens are in the impressionable and tender first few weeks and months of their lives, they learn a lot through activities such as rough playing and mutual grooming. These early times are key to a kitten's proper social development. Because of these important learning experiences, the Humane Society of the United States advocates keeping kittens with their mothers and littermates for a minimum of three months.