It depends. Most women miss their periods two weeks or less afterwards. Morning Sickness usually occurs at about 6 weeks. Depending on your size, the average women starts showing at 3-5 months, but I've met women shaped like sticks who started showing at two months.
Every woman is different. Most start developing signs within a couple weeks. Many are like a period getting ready to start. If you are worried about being pregnant, it can cause you to be seriously late even if you are not. Once you are late, take a home test. If there is enough hormone to make you not start there should be enough to show a positive test. Home tests are very accurate if used correctly. If it shows positive, it is positive.