How long does it it take for the effects of aspirin to kick in?

5-12 minutes. A study published in a cardiology (heart) journal found that Aspirin started working (preventing platelets from clumping) after 5 minutes for chewable tablets, and after 12 minutes for tablets swallowed whole. If an adult takes Aspirin for pain or fever, probably more time needs to pass before he notices less pain or lower fever, perhaps 30 minutes.
A couple hours. It can take a couple of hours to feel the effect of Aspirin for pain relief. The anti-platelet effects of Aspirin last for seven days. Ibuprofen and Naproxen are better anti-inflammatory pain medications than Asprin.