I'm working on a cardiac step-down unit in a moderately large city. The orientation on our unit is typically 6 to 12 weeks with 8 being the average. Our unit's educator, who oversees the orientation process of all of our new-hires, decides when she believes you are ready to fly on your own.
It may sound intimidating but I think it really depends on the kind of support system available on your unit after you start on your own. Our hospital has charge nurses who don't take patients and who are made fully aware of who is newly off orientation. As a result they know to kind of hover over you for your first weeks on your own.
A majority of new nurses also begin on nights. This is good for a couple of reasons. First, there is typically less going on so the atmosphere is less overwhelming (no doctor's rounding, meals, physical therapists, case managers, etc.) so they can focus on improving their practice before moving to days. Second, less going on frees up more experienced nurses to be available to help a new nurse in need.
In any case, I wish you the best of luck and welcome to the profession!