How long are lunch breaks supposed to be?

Perhaps you should ask your company for their policy.

Some jobs provide 30 minute lunch break while some provide full hour break.

I have worked at both. Strangely even if it was same work hours, I received hour lunch, yet with another employer 30 mins.

If your work timing is 9 hours, then it's likely going to be hour lunch. Otherwise it's overtime hours, if these are your timings Monday to Friday

1. Please pay attention to the category your question is automatically sorted into, as it may not be the correct one. You can change it.
2. That's something to be worked out between yourself and your employer. A contract must be something all parties involved agree to. Your employment is a contract. You agree to the hours, the workload and the pay.

The minimum lunch break depends on the local labor (labour) laws (state or province usually).  Generally speaking though, the company has a policy that could provide more than the legal minimum.  A half-hour would be a typical minimum lunch break for a work period of more than about 6 hours (details can differ).

Normally, an eight-hour workday has a 30 minute break for lunch if you aren't in an office setting.  It is primarily a safety/health and welfare type of requirement.  You ought to ask your supervisor, your boss, what the policy is.  They clearly know that they must give you breaks, unless perhaps you are working in some third-world nation where labor laws are essentially non-existent, and temporary assignments do not count as working in that other nation.  The laws that apply in your usual place of work are what would apply.  If you are hired by a US office of a multinational and get sent to the Congo for 3 months or something, the US labor laws apply to you because you are a US employee, not a Congolese employee.

As per your comment on another answer, I agree that your lunch break should be an hour. You're supposed to be working an 8 hour days, not 8.5 hours. Essentially this company is scamming their employees everyday.

The way things typically work is that lunch is an unpaid break and any tea breaks provided (if you receive them) are paid. Therefore even if the company allows 2 x 15 min tea breaks (which sounds unlikely), they can't just add the time onto the work day.

half an hour, not much shorter