I agree with Penguin. Sounds like you need to do a bit more research on cycling the tank. There is no specific length of time that you cycle it for - you cycle it until the cycle is complete - usually 4-8 weeks. The *basic* principal is that fish produce ammonia, which is a deadly toxin. There are bacteria that convert the ammonia into nitrite, which is also deadly. Luckily, there are bacteria that convert the nitrite into a much less harmful chemical called nitrate. When you cycle the tank you are basically growing these bacteria in the tank so that when you add fish they will do their job of converting the ammonia into nitrite and then into nitrate. In order to start this process you need to add a source of ammonia to the tank so the bacteria you are trying to grow have a food source, and you will need to continue to add ammonia until the cycle is done. Typically once the cycle starts you will get ammonia readings, then eventually you'll start getting nitrite readings and the ammonia will start to go down, then eventually you'll see nitrate readings. When the cycle is finally complete your test readings will show 0 ammonia, 0 nitrites, and readable nitrates.
Do a search on "fishless cycle" and you'll find lots of information.