My son's father is an absentee father. He hasn't seen or spoken to him since July of this year when I allowed him to visit for the summer. He seems to not want him in his life. I was wondering how long I have to wait for my fiancŽ to be able to adopt him without any complications.

The complications your talking about will be the current fathers willingness to give up the child or fight it. As for the adoption process you should be married first and for at least 1 years time. Then you can get things started.

He never just loses his rights automatically by not visiting the child. Do you have court ordered child support. That often helps a absent father decide to sign over his rights.

But I will agree after you are married for a year, you have an attorney start adoption proceedings, the bio father will have to be notified, and asked to sign his right over.
He may sign, he may show up and fight, or he may just not sign but not show up, where you prove it is to the child's best interest.