shadowing- how long should I stay?

I had an interview last week and the manager asked me to come in tomorrow to shadow. He said I could stay the whole day (7am-7pm) or a half day. I'm really nervous and was wondering first will I be doing any patient care? Legally I would think I'm allowed to because I'm an RN and am employed by that hospital already, but not on that unit. Also would it look like I'm not interested in the job if I left around lunch time? I really want to get this job, I just think it's a lot to have me shadow for 12 hours! Am I being unreasonable?!
Looking forward to your responses! Thanks!!