So, your kittens are finally eating solid food. Watching them play and discover the world is hypnotic, but you've got to keep an eye on mama cat. After her kittens are fully weaned, she should stop producing milk fairly soon. If she doesn't, a problem may exist.
Weaning is a process, not an event. Kittens begin eating solid food when they're 3 or 4 weeks old, but most aren't completely off mother's milk until they're around 8 weeks. Toward the end of nursing, and sometimes even a couple of weeks after it, kittens may make sporadic attempts to feed on their mother's teats. During the height of lactation, the mama cat will probably eat voraciously. As this stage wanes, her appetite should likewise drop off. Her breast will likely shrink as her milk dries up within two weeks. Don't wait for her next estrus to tip you off -- that could be a month or two away.
Even with the extra weight afforded by milk, a lactating mother cat may not be as large as she was before she had her litter of kittens. Still, the hormonal changes that accompany nursing kittens may change her behavior, and even her appearance, rather drastically. The mama cat will likely stop lactating by the eighth week after the kittens are born. As soon as the kittens appear to be eating nothing but solid food, the mother cat's breast should be dry within a week or two. Typically that's week 10 for the kittens.
What to Watch for
Weaning is tough on a mama cat. Her eight mammary glands may appear red and swollen during nursing and weaning, but they should heal. If her teats look like they're getting worse after the kittens are through with them, she could have a medical issue. Sometimes this will look like there's still milk in her breast. If her stomach's hard to the touch or the milk is discolored, that's a sure sign somethings amiss. If it seems like your mama cat shouldn't still be lactating -- or if anything else is out of the ordinary -- contact your vet.
As a general guideline, kittens should stay with their mothers until they're 12 weeks old. A big part of that's because of socialization -- if a kitten shouldn't be feeding on his mother any more, the mother will let him know. Still, this could stimulate milk production if she isn't dry yet. Hormonal imbalances could also prolong lactation. It's possible a mama cat can continue lactating because of another pregnancy or a pseudo-pregnancy. At the far end of the spectrum, a mother cat may continue to have a swollen breast because of a tumor, benign or cancerous. If you're concerned, call your vet.