When Is The Best Time To Give My Cat A Bath After Having Kittens?

You don't need to give her a bath, she will clean herself up. If you wash away the way that she smells, it might be hard for the kittens to find mom when it's time for them to nurse.
Since kitten's eyes don't open for a couple of weeks,  little things depend on their sense of smell to find mom. If you change that, it could hinder them severely. Good luck.

Most cats never need a bath, including new mothers. But those with skin conditions (such as ringworm), that have soiled themselves with diarrhoea or by rolling in something foul, and cats at the end of chemical flea treatment need to be bathed.
Also, elderly cats lose the ability to clean themselves, necessitating occasional baths. But younger cats that
need to be bathed frequently because of greasy fur may have a health problem or be on an improper diet.
Gather up cotton balls, Vaseline, several towels, a hand-held hair dryer, pure Castille shampoo (from a health-food store), and a hose that connects to the kitchen sink or bathtub faucet.
First, groom the cat to get rid of loose hair and dirt. Mix a solution of half shampoo/half warm water in a squeeze bottle. Start running water in the sink or tub at the temperature of a baby's bath, keeping in mind that a cat's body temperature is higher than ours.
Meanwhile, stick half a cotton ball into both of the cat's ears, to avoid possible infection from soapy water.
Then apply a small smear of Vaseline to the cat's lower eyelids; the oily film will protect the eyes from splashes.
Arrange the hose so the water is running down the drain then gently place the cat in the sink. Talk constantly to him with your face close to his to reassure him.
Holding the cat firmly, gently run the water over his thighs, lower back, sides, inner tights, upper back, shoulders, throat, and chest -- in that order, so he will get used to the water sensation gradually. Do not attempt to wash a cat's face.
Apply the warm shampoo mix in the same order as you wet the cat, and lather him up. To rinse, hold the hose at its tip, never letting the cat see water flying at him.
Squeeze the excess water out of his coat then dry him gently with the towels. Finally, after showing the device to the cat, use the hair dryer, set at a warm temperature, as you comb his fur with the other hand.