It varies too much depending on difficulty.
After an initial phone/email conversation, I can come to their home for a free consultation, or they can email me photos of the space. During this time, I will develop ideas for the room and I will email samples until they are completely satisfied. These samples will be superimposed pictures onto the photos they send me, this way they can see almost exactly what the finished mural will look like!
I have an associates degree in graphic design.