How long after unprotected sex do you have to wait to take a pregnancy test?

The last time my ex and I had sex, before I found out he has been seeing someone else for 8 months, was Saturday, July 31st. I don't have regular periods and haven't since I was a teenager/before I had my 18 month old daughter. Thanks.|||usually 10 to 14 days you can say around 2 weeks|||Well its hard to know when you have irregular periods, but if you count 28 days after your most recent period began, that should tell you when your next one is due, roughly. buut they reckon you should do it the day before your period is due. Or any time after that. the way it works is it picks up the hormonal change in your pee. :) But don't forget that they can lie about a negitive but not a positive.|||Wait until 28 days from your last period. Use an early result test if your not too sure, they detect the hcg hormone very early. I used one and found out I was pregnant 6 days before I missed my period and I didnt even use my first morning urine.