When is it ok to give my baby juice?

Rarely if ever. I am not a big fan of giving any juice to babies or children since most juices are just sugar and water. Babies should drink breast milk/formula until 6 months, then water can be added. I also discourage juice since once juice is given, some babies won't like their breast milk/formula/water as much. If you are going to give juice, i definitely recommend diluting it as much as possible.
Only if needed. Juice can be given after age 6 months, if recommended by a doctor to help with some problem. For example, apple juice can help relieve constipation. Otherwise, normal healthy babies do not need juice. They can eat fruit, prepared appropriately for the age of the baby, once they are 6 months old.
Six months. Six months is when juice and water are generally started for infants. However, juice (which is mostly sugar and water) is not a necessary part of infant nutrition and most pediatricians (and dentists) would be quite fine with never giving it to infants.
After eating fruit. If your child has eaten the fruit then it is okay to drink the juice. Just remember that juice is junk food for babies. Juice is high in sugar and should be limited to 4 ounces daily.
When someone... ...Figures out how to make juice without throwing all the good stuff away with wrung-out pulp. For now, eating fruits and drinking water is you best bet, from birth to past retirement age.