1)
What are you doing now? I'm watching a film.
When did you start watching it? I began watching it at 3 p.m.
How long have you been watching it? I have been watching it for two hours.
2)
What are you doing now? I'm reading a book.
When did you start reading it? I began reading it two days ago.
How long have you been reading this book? I have been reading it for two days.
3) What is she doing now? She is solving a puzzle.
When did she start solving a puzzle? She began solving a puzzle two hours ago.
How long has she been solving a puzzle? She has been solving a puzzle for two hours.