How long do I have to stay at a job so it does not reflect on me negatively?

I recently started a Pool position at a hospital that a few nurses I have worked with before recommended. They all said they love it there. unfortunately, I do not. The charting system is horrific! It is 45 minutes from my home. and even though the nurse/ patient ratio is fair, the nurses there are always complaining! I just don't like the vibe there, if you get what I mean. I just finished a 2 week orientation. and to be honest....I don't like it there. I still have my other job where I have been for 5 years. I was thinking of leaving because the staffing is not safe. so, I took this job in hopes of a change for the better.
My question is this.....If I leave now...how bad does this look? If I ever wanted to get a job with this company again (they own many hospitals in my area) will it be held against me leaving right after orientation? I have been a nurse for 25 years and have never done this! How long do I have to stay for it not to reflect on me in a negative way? thank you for any opinions!
If it's a true PRN position, you could just gradually "phase out" by working once in a while and then declining due to schedule conflict (with your primary job) when they ask you to work a shift. Most organizations have an automatic "clean up" process to remove people from active status if they have not been paid for a specific period of time and are not on an unpaid leave of some sort. I have seen ranges from 3 months to 12 months on this inactivation process, so it may not happen for a while. They will probably contact you when you get 'inactivated' to let you know. In my experience, this is usually recorded as an "eligible for rehire" unless you have ticked someone off along the way - LOL.
HouTx, Thanks for the advice! I did think of your suggestion.....I told them I wanted to work 1 or 2 days a week....the requirement is one per month.....so I could just work the rest of this schedule and sign up for less time on the next one...and then none. :P My other problem is a few of my former coworkers work at this hospital, and I know they will be facebooking me to see where I am. They think it is so much better there then at the other hospital. Like I said the staffing is much better....but I feel like a secretary with all of the paperwork! I am wondering if they got sued at one point with all of the different redundant things that need to be filled out. Thanks again!!