Hi LisaLi hope all is well!
Jeremy ran the current course Welcome to forums.fast.ai over an 8 week periood. However in the book there 20 lessons/chapters the course covers 8 of these. Jeremy advises A week of study per lesson.
Depending on factors such as time, previous knowledge and skill,I have seen people take 6 months to a few years to master the material. The book contains many furtther reading exercises.
However may people who have some programing experience or aptitude build their first image classifier in a notebook, in tthe first week or two and many of these are in the form of a simple app. (Share you work here - highlights)
Hope this helps
mrfabulous1 