When will it be a good time to plan for a 2nd one?

3 years will be a great age gap as elder one will tend to be more understanding. Any age gap will still lead to sibling jealousy so personally I feel it is up to parents preference and how ready you are to accept another child into your life. My kids are 5 years apart and my elder one is still jealous of his little brother at times but I get a lot of help from him like helping to put the laundry into the laundry basket, fetching stuffs etc.