when i can apply visa again?

my visa issued june 18,2012 until september 18 (just single entry),i went to china june 21,2012 and have 14 days duration to stay ..i came back july 6,2012,, i just want to ask when i can apply visa again? can i apply this september or october this year? because i want to go back to china during winter,,,thank you hope you can help me..have a good day..