How long after you leave a job does your health insurance is supposed to expire?

My mother says by law, it should cut off 3 months from when you left the job. Does any know the legal side of this?
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It may depend on how the company has set up the plan, but every place I've ever worked, you're covered through the last day of the month you leave. So if your last working day were today, for example, you'd have coverage through May 31. After that, you can pay the premium yourself, the same rate the company pays, (called COBRA or State Continuation), for up to 18 months.
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