How long must a father be absent before his parental rights can be terminated and the children adopted by their stepfather?

After a year, I'd say you are just now getting into the territory where you can start actively thinking about this, unless the children are still very, very young, no more than 3 or 4 right now.  If they are still young enough that they don't have any really well-formed memories of the man, I'd say a year is enough to go ahead.  I'd also expect that you and your present husband would be together for at least a year before you start the adoption;  while it isn't strictly necessary under a statute, you'll need some basic showing of stability in your own situation.