I got one about 4 days early, but that doesn't always happen. Some people don't get a positive until a week or two after they missed their period. You should test when you miss your period and if it is negative, again a week or so later if you still have not started your cycle.
You should also check the test to see what the sensitivity is. The lower the number the better the test. (Example: 20 ml is a higher sensitivity to the hCG hormone than 25, or 50, etc.)
Last, but not least, always remember that everyone is different. My first pregnancy never gave me a positive urine test.