I've never been one to keep track of cooking times with meats, since it will vary wildly with meat thickness, burner strength and type, phase of the moon, etc.
Edit: I forgot to answer "how to go about searing". I sear chicken like I sear beef: hard and fast. The point is to get that Maillard reaction going to add some deliciousness and texture (not to "seal in flavor", which is hogwash).
- Pat the chicken dry with paper towels.
- Put just enough oil in a heavy-bottomed pan to cover the bottom.
- Put the pan over medium-high heat and get it good and hot - the oil may just start showing wisps of smoke.
- Lay the chicken in the pan carefully, being sure to start at the edge closest to you and lay it down away from you. This will prevent you from getting splashed with hot oil.
- Let it cook for 2-4 minutes until you get a nice sear on it.
- Flip, (the chicken, not you) again being careful to flip away from you.
Now if the breast is thin enough (maybe you butterflied it beforehand), you can just let it finish in the pan. Often, though, after flipping I'll pop the whole pan into a 400F oven and finish it in there. Again, times will vary, but I would start checking it after 5 minutes.
The most accurate way to determine doneness of any meat is with an instant-read thermometer. I love my Thermapen, but it's a bit pricey. You can find inexpensive dial or digital ones at your local grocery. The recommended internal temp for poultry is 165F.
Normally, I poke my chicken with a finger to determine doneness, then double-check it by cutting it open. Fully-cooked chicken is white all the way through, and the juices run clear when you cut into it. Under-cooked chicken is pink, and the juices run pink as well. I get fresh chicken from a local farm, so I cook it until it is barely done to be sure it stays moist and delicious. For supermarket chicken I would err on the side of completely done, since you have no idea where the meat came from.
For reference, fully cooked chicken:
Under cooked chicken:
Extremely under cooked chicken: