6 weeks... Kind of...
The Family Medical Leave Act (FMLA) allows for 12 weeks of unpaid, job-protected leave for eligible employees who work for companies of 50+ people. Any health insurance plan that you're enrolled in must continue as normal This is national law. The tricky part is eligible employees. There are a bunch of rules that depend on how long you've been with the company, how many hours you work per week, etc.
If your company is less then 50 employees then you have to look at state law. Just google FMLA in (your state). Here in MN the law says that companies of 21+ people must offer 6 weeks of unpaid, job protected leave. They must keep you on the health insurance as normal but you may be required to pay the full premium. If the company is less then 21 people then the company can basically do whatever it wants.
That's the other tricky part. Each company may offer a different policy so you need to check with them. Some offer paid leave, some offer longer periods of time, it all depends. Each state has different laws too... so that makes it even more complicated. That's not even taking into account loop holes that some companies magically find.
I was offered 6 weeks unpaid, with an additional 2 weeks part-time to ease back in if I wanted. I had to use up all of my accrued PTO first. It was nice to get paid for that time, but it would have sucked to go back without any vacation/sick days. When DD was 4 weeks old I decided I wanted to start my own business and work from home so it didn't matter anyway.