3 Answers

Depends on the number you are cooking and if they are breaded or not and what temp you are cooking them at. I would say 425 degrees for about 30 minutes for unbreaded and up to 4 spaced apart. Breaded I would cook at 325 for one hour. As long as the breast is white all the way through, it is done. 

You cook them until the juices run clear. Time depends on how you are cooking them: fried, poached, baked, etc.... Start with 20 minutes and then start testing them....

Buy a meat thermometer, the best and most accurate way, nothing worse than dry,stringy chicken, make sure they are cooked but not overdone,don't forget to rest them before serving (unless crumbed and fried).I like to cook them in a wine or lemon sauce, keeps them moist.