Two weeks notice. Is it something that needs to be done all the time when leaving a job?

I want to leave my job. In short I can’t trust my boss. For example, I was told I would be covering for someone’s maternity leave and then learned today that they’re looking to replace this person before the maternity leave comes into play.

I’m not making enough at this job to live off of anyway and she knows it. So should I give her the benefit of any notice? Or is it OK to (more politely than this of course) shove the job up her ass, find someone else to cover all of the work I was already set to cover (different than the maternity leave), and just leave?