Work time items can be added to product structures and sold. The unit of work time is called a "period" and is system wide. It is specified in the system settings as number of minutes per period. So to use hourly timings, specify 60 minutes per period etc.
In general it is only necessary to specify work time items for each cost that is in use in the system. For instance if there is "Labour standard" and "Labour overtime" with different costs, create different work items. Similarly if "Welding" costs differently from "Assembly" use two different items. In general it is not recommended to create work time items per person.
Most importantly check that the cost specified is correct for the period setting of the system.