Maximum length of stay with an L VISA

Hi, I've just received a job offer in China. My company would like me to fly over with an L type because I won't have my degree notarised in time - the intention is to have it changed to a Z type once I am there and have it notarised back in the UK. I was just wondering what is the longest period I can stay in PRC with an L type? I'm sure the change will take less an month but it depends on how fast my university send my certificate but i was just curious as I really don't want to have to fly back to the UK because of costs etc.