L visa query

Hi there. I am arriving in Shanghai on October 11th to backpack in China solo, and will visit several cities and provinces. I am just in the process of applying for the visa now and I am perplexed by the exiting flight conundrum. I'm reading a lot online that often the l visa will be 30 days long. However, my itinerary would be a lot more flexible if I was to have around 40 days. I am hesitant to book the exiting flight without knowing how long my visa will last. Do you have any advice? Should I book the exiting flight for no longer than 30 days after entry? Thanks for your help.