How can I help someone with post natal depression? And how long of a period does it last?

There is no specific period, best thing to do when being a caregiver for someone in such a mental state. Provide lots of support, emotionally, mentally and physically. Never under estimate what they are going through. Love and patience will also help the person. It is not easy being a care giver. As a care giver do not forget to care for your own well being. With support of family and friends it will get better, slowly but surely.