There are no defined "rules" for what a Skype interview will look like. However, to give you some pointers:
How long will it be?
You should ask the interviewer. I have had Skype interviews between 15 minutes and a little over one hour. If the interview would go significantly longer than that, I would assume you would have been told.
Will it be a Q&A session? In that case, what type of questions are usually asked?
Presumably there would be Q&A, yes. Questions could be any kind of typical interview questions, i.e., they could relate to your research, your teaching, how you advise students, what grants you are interested in applying for, or anything else really.
How to react if the position is offered directly and the applicant wants some more time to decide?
"I am very happy about that. However, I will need to get back to you within [X timeframe] as I need to discuss this decision first with my wife."
(no, no reasonable PI should expect an answer from you right now; even though they may indeed offer you the position immediately, you are under no pressure to accept immediately)