How many times you can exit China depends on the kind of visa you hold. If your visa is a 'multi entry visa' then you can leave and come as many times as you want until the visa expires. Now, if you have a tourist visa that's...say...2 entry...then you can only leave and come back 2 times during the validity of your visa. Maybe the visa should last for 6 months, but I leave once after 2 months....if I leave again before the 6 months is up...I can't return to China unless I get a new visa (easily done).
As long as your visa is valid and you have entries left, you can go to Japan for 3 days and then come back. In doing so, that also resets your days stayed in China back to 0...so you need to leave again after a year is up.
I am back in China after getting my new visa in Hong Kong. Hong Kong was AWESOME
and a nice break from the mainland. I think you only need to leave china for 24 hours, but it took me 2 days to get a new visa (the application day and then the next day...I paid the rush fee).
I hope this is clear and understandable. I'm really tired right now. Little sleep plus I just finished 2 classes. If you have more questions you can message me on MSN or Skype.