First time opportunity to give a seminar
If you have presented your thesis before, the seminar could be something similar to it. However, a seminar is usually given to a wider range of audience, and thus you need to make it more accessible for someone in the audience who may not be familiar with the subject. Aim for an accessible introduction, and save the technical details to the second half of the talk.

As for how long the seminar should be, you need to ask the professor how much time you have to talk, and how much time you have for questions and answers.