On a medical note, even a woman who breastfeeds fully and has not yet menstruated may conceive after six months. (See our article on the Lactational Amenorrhea Method for more details.) Your wife’s nursing will not necessarily delay conception.
Halacha encourages breastfeeding, especially for the first two years of life, without concern for possible reduction in ability to conceive during that time. Halacha does not recognize desire to speed fulfillment of the command to procreate as reason to desist from breastfeeding, especially because breastfeeding itself is a form of involvement in and giving sustenance to a child. Breastfeeding thus may be considered part of fulfilling the command to be fruitful and multiply.
You should feel free to discuss your desire to have another child soon with your wife. But you should be careful not to pressure her.
In any case, six months is an appropriate age to begin supplementing breastmilk with solid food. Even if both you and your wife are eager to conceive again quickly, there is no need to start giving formula.
Updated August 2020.