There is no way to predict how long jury duty may last. In some cases, being called for jury duty doesn't involve getting picked to sit on a jury, and the process is over within a few hours.
However, if a person is picked to sit on a jury, she could spend weeks or even months performing jury duty. This is especially relevant in complex cases such as murder trials or lengthy lawsuits. If a person is selected to serve on a grand jury, her service could run for a specific amount of time, and she could hear numerous cases while serving.