Different schools have different rules. At present, the undergraduate education system is generally flexible and four years is normal (except medicine and architecture). You can apply for a five-year extension three years in advance, but it is a very sad thing to postpone it without justifiable reasons. After all, any unit or enterprise will ask if it is delayed to graduate. As for studying for 6 years, please consult the school. Whether the school can graduate or not, you need to report the graduates to the education department for advice.