Employees should always know what their rights are as an employee or they can be taken for a jolly ride by the employer. Some offices do have good work practices, but some do not. When it comes to taking breaks, employers can be really rigid or nasty about them.
Most employers do have standard and fixed timings for the breaks for the employees. However, the Federal law does not mention that an employer has to provide meal breaks. Employers who give the breaks do it out of ethical reasons. Also, if you get a meal break, the employer does not have to pay you for the time you take the break. Some states in the US do have different rules and they require the employer to give meal breaks. The Fair Labor Standards Act that governs these matters is different in several states.
If you have a break, the employer does not have to pay for it unless the state law mentions it. The state laws may sometimes mention to pay paid rest or meal breaks. Sometimes it is mentioned that you will need to work right through the break. Your break has to be 20 minutes or less and they have to be paid. There are not many States in the US that follow the rules of meal breaks. In most cases, it is assumed that you are not being paid for it. However, this rule is not mentioned when you agree to the wages with your employer. It is assumed by the employer though.
In order to know what your rights are visit the States employment laws website and find out.