Not a stupid question at all.
It depends on what kind/stage of the menstrual cycle you guys "had a go at". It also depends on what kinda of test you use and what enzyme/hormone you are looking for.
The earliest you can detect is within 48 hours of fertilization - using the EPF (Early Pregnancy Factor - it's a protein associated with mammalian fetuses). However, the EPF is an expensive test and requires a blood draw.
The other - and more commonly/widely used - test is the hCG (human Chorionic Gonadotropin - a hormone) test. The catch is that hCG usually cannot be detected in the urine (or blood) untill the embryo undergoes implantation on the uterine wall --- which means a lag of a good 6 to 12 days post fertilization.
So you are looking at a week to 2 weeks (if using the commercially available Early Pregnancy test kits). Don't forget to get confirmatory testing if the test kit shows positive.
And please start visiting a doctor to start maternal pregnancy regimen and pre-natal care. Iron, folate and other dietary needs are vital at this time.
Hop to it, young grasshopper!
cheers,