How Long Should I Wait To Check Whether I Am Pregnant After Missing Periods ?

Hi, I had a baby in 2009, I was breast feeding her for 1 year and till then I dint get my periods. When I stopped breast feeding the periods have started but they are irregular, either early or late by 2 or 3 days. Last month I had my periods on 07.05.2011, and had sex on 10 and 11th day of the menstruation cycle, thinking it is safe period. Now, today it is 7th and I missed my period. I dont want to get pregnant now. How long should I wait to check whether I am pregnant?