What may keep you in the hospital is the complication leading to the swelling of the scalp
. The procedure of removing the ommaya reservoir itself is uncomplicated within reasonable limit. However, you will have to stay in the hospital longer than if there were no complication. This is for monitoring purpose and treating of any underline condition. For example, infection if diagnose. So you may have to stay for about 5 days at least. Surgery are plan considering priority and surgical priority is given with respect to severity, so you may not have to wait long.
Wish you good health. Hope this is helpful