Not every job requires a look at your background but most jobs dealing with security or children do. The length of time and depth of a pre-employment background check can vary from job to job and so the reliability of criminal background checks can also vary depending on how far back and what methods the employer uses. Its a process you either must do, must be willing to submit too or that has no affect on your application what so ever.
A pre-employment background check can take anywhere from one day to one month depending on the employer. Most employers will actually hire the investigative work out and not use their own in house investigation to acquire the information that they require to further your job application. The information can be anything from the basic court house records of what you have or have not done in the past year, to a full comprehensive look at your past jobs, residential history and anything else they can dig up.
In many cases the failure to divulge information prior to the search will result in an automatic withdrawal of your application for employment. As a general rule the history checks will take no more than a week if it is not an in depth look into your history and the reliability of criminal background checks is very accurate, so it is not worth hoping that something will be missed. You may also ask the interviewer how long the background check may take; they should be able to narrow down the time frame more accurately.