You take it for 15 days to kill any larvae that hatch in the meantime. Eggs laid the day before you started take about two weeks to hatch, so you have to keep it going until those last larvae are killed. The ivermectin makes the adult worms sterile (doesn't kill them), so they won't be laying any more eggs, and if they do, they won't be viable.
Plus, if you have strongyloides, you won't see a lot of anything in your stools. I did at first with the Ivermectin, but just a few roundworms and what looked like some ropeworm. All I have now is a few worms coming out of my skin, but pretty much nothing visible in my poop. Strongyloides are so tiny, you won't see them in your poop at all.