This depends on the role(s) that you are interested in. Some roles are a short term and ask you to commit to a couple of hours or one day. Others might ask you to commit to a few weeks or months.
Most organisations will only ask you to commit to a couple of hours a week, on an ongoing basis.
If volunteering with vulnerable adults or young people, the organisation will generally ask you to commit to a longer time period – perhaps six months to a year. This is to allow time either for a lengthy screening process or to give you and the service users time to build up a relationship.