How much should I spend when dealing with my new girlfriend and her 6-year-old child? She is putting unrealistic demands on me. I never wanted children and don't really want to change my entire life around because now there is one around.
My short answer is this, if you don't like children RUN now before you get yourself in too deep. If you are considering a long-term relationship with this woman you better prepare yourself to share every day of the next 13 years (until the child is 18) with a developing young person who will require, and possibly demand, a whole lot of time, energy and resources.
Partnering with an already-parent is difficult. I have worked with many couples in your shoes. Some new partners in your shoes struggle with "Who am I to this child?" and "What are my rights and responsibilities when it comes to parenting?" Or "How am I supposed to deal with this weekly contact between my partner and her ex/co-parent?"
I encourage you to have very frank conversations with your girlfriend, letting her know your expectations around the parenting issues and stay grounded in what is real, not what you'd like to be real.
The fact that you don't want children is something I suggest you pay attention to. When you partner with someone who has children, it is a package deal.
It sounds like you are not comfortable with the reality of your situation. No matter how disinterested you are in parenting, when you choose to partner with someone who has a child, you are choosing to involve yourself with a child - even if the agreement is that you have no parental responsibility for him or her. Are you prepared to be in this child's life for the next 13 years?