What visa should I get

I have have one entry left on my double entry permit which is valid for 2 months. After this I want to come back to Mongolia then when I finish my travels in mongolia get another Chinese entry permit. What is the longest multiple entry permit I can get? PRC is huge and I want to spend a lot of time exploring each province in depth not just the normal quick tourist trip. As I have already had one Chinese entry permit is the next one able to be Longer? In Australia I was told for my first entry permit it was hard to get anything beyond a double entry one for my first trip. But on the second I could get a much longer one.