It depends on many factors.
1. How skinny you are (how much muscle/fat do you have going into the pregnancy),
2. How aware of your body you are,
3.How many pregnancies you'e had prior to this one,
4. How far along you are.
So, typically if you're skinnier, you tend to show quicker, and it will be much more noticable on the outside through touch when the baby is kicking because you'll feel the muscle spasm. Otherwise, it's up to you to know your body and when it's just the uterus expanding and spasming or the baby (because kicks are few and far between at first). If you've had multiple pregnancies, your uterus expands much quicker (you show earlier and earlier for each because the muscles already know what to do). And this will also predict when you feel it. Some have sworn that after multiple pregnancies they could feel baby between 9 and 12 weeks. People who don't have multiple pregnancies usually feel it between 18 and 25 weeks along.
With my first I felt him at 18 weeks. With my second I felt him at 14 weeks. (These were pregnancies 2 and 3 for me). I'm only 7w4d today, so still have to wait to confirm if I can feel it any sooner than that. But this is pregnancy #5 for me.