How soon after intercourse can you take a hpt not knowing when your next period should be?

1 week. You can do a pregnancy test 1 week from last intercourse. If negative then chances of being pregnant are pretty small.
Three weeks. Do a home pregnancy test three weeks after the episode of sex in question. . If the result is negative repeat the test 4-5 days later unless you get a period by then. Use first morning urine and follow the instructions for the test carefully.