How long should you wait in between fertility treatments after having one baby?

1-2 years. Typically 1-2 years is adequate. It also depends on several other factors such as breast feeding, medical conditions, social situation, ...Etc. Also it depends on the couple's readiness for another pregnancy.
It depends. If you have a completely normal pregnancy and a vaginal delivery, then 6 months is the minimum waiting time. If you have a cesarean section then 9 to 12 months is better. If you do not wait, then the complication rate for the next pregnancy is increased. If you had a complicated pregnancy or any problems after delivery, then you should wait longer. Please discuss this with your ob.