This is a picture of one of my first models, little Alexandra. She was only five when she "sat" for her portrait - and it was then that I realized that five-year-olds don't particularly appreciate all the time and effort that goes in to modelling a bust...
Ideally, I like to have the subject visit the studio several times over a period of weeks. During the first couple of sittings I take pictures and make measurements that allow me to do a lot of the modelling without the subject being present. This helps minimize the amount of time the sitter needs to spend in the studio. During the final finishing stages of the work, however, there's really no substitute for having the sitter present.
Sitting still for a long time is not easy. After a little while, most sitters tend get restless - as if they were waiting on a red light in traffic. Fortunately, I've found that by letting people watch movies while I work they can happily sit still for hours at a time.