Multiple entry visa type L entry and re-entry wait time

How long do I need to be in Hong Kong before I can come back to the Mainland??
I have a multiple entry type L type to china. I am an American and my L type is valid for 10 years. I want to stay in China a month longer but don't want to go through the hassle of applying for an extension. I'm visiting near Hong Kong and was wondering if I could just exit the Mainland and then come right back or if I need to stay a day or two?