Usually, they get quite strong by the end of the second trimester. If you have an anterior placenta, it might take a bit longer to feel them clearly, so check with your doctor if you have doubts.
Oh, by month 7 or 8, you'll feel like they are practicing football inside your ribs! Enjoy the gentle flutters now because soon you won't sleep from the constant kicks.
The kicks got really strong around 25 weeks for me, and it was such a great distraction from my endless nausea. Hang in there, those sweet thumps are coming soon!