How long?

The orientation period is a time for both you and your employer to see if you are a good fit for the position. If, at the end of the orientation, you are sure that the job is not for you, it's best to leave before you start working on your own.

This is, of course, assuming that your orientation is long enough to give you a decent feel for the position. I would say that after a solid month on the job, you should have learned enough about the position to know whether or not you love it, hate it, or somewhere in between. But I don't think there is a one-size-fits-all answer here. Every person and every job is going to be a little different. Trust yourself. If you know that you have given the job the best shot you could, and that you have been their long enough to really know it's not going to work, then that's when you should leave.

It's certainly not wrong to work at a position for awhile and then leave if it's truly not working out for you. It happens all the time. The most important thing is that you keep the interaction professional. Don't bad mouth the facility or list all the reasons you are leaving. Just simply state that you don't feel you are a good fit for the position and need to seek other opportunities. Give them adequate notice- two weeks at the minimum. Thank them for giving you the chance to work for them and express regret that things did not work out. Don't burn any bridges, and you'll be much better off for it.