You should be producing enough of the pregnancy hormone for a test to pick up 2-4 weeks after conception. It depends on the type of test too but the early detection ones are more sensitive. I would test at around 3 weeks after ovulation, then once a week until the next cycle starts, but I am very impatient lol.
If you have that long of a cycle test four weeks from your last period.
That would be about 8 weeks from my last period :(...
I wouldn't bother testing until you have a missed period. Maybe when you are about a week or two late.