When hiring a genealogist, you should realize that genealogy is never-ending. A hobbyist can spend a lifetime researching their family history, but unless you have a never-ending supply of money, you will not want your genealogist spending forever on your family tree. You will need to have a goal in mind for your genealogist. Some goals could be:
I want to know the names of my immigrant ancestors.
I want to find my birth parent/grandparent/etc.
I want to know where my ancestors came from.
I want to join a lineage society.
I want to know about my military ancestor.
I need assistance to break through a genealogical brick wall.
You will also want to provide your genealogist with relevant information regarding your goal(s) and any research you have done on your own. With a goal in mind and prior research, your genealogist can provide you with the scope of the project. A genealogist cannot predict the hours needed to research your goal(s), nor will they be able to guarantee they can prove ancestry. A great genealogist, however, keeps you informed along the way so you may decide which avenues to pursue and how much research you can afford.
The Association of Professional Genealogists (APG) and FamilySearch have great advice on hiring a professional and the standards to which genealogists are upheld. APG also provide a wonderful directory of genealogists that you can search by name, location, research specialty, geographic specialty, and more. I hope you consider Flipping Through Records when you are ready to hire a genealogist.