Forever
I spoke to my stepson once when he was 8 years old about how he felt about his parents not being together. He said it was totally normal, there wasn't a single kid in his class whose parents were together.
His answer shocked me but also I realised that the idea of family is so different now then when I was his age
As long as your kids are loved and know they are wanted they will be fine. It sounds like are in a good place with their dad and they won't be exposed to the two of you fighting, which is very painful for kids.
Try not to dwell on what could have been. Enjoy what is. Your broken heart will heal and your new family structure will evolve over time.