When a rollup field is created, behind the scenes, a system asynchronous job is scheduled to execute 12 hours after the field is created. Navigate to Settings | System Jobs; you will see a job of type Mass Calculate Rollup field with the name of your new field in System Job Name. You can force the job to run sooner by clicking on Action | Postpone and entering an earlier date/time. It is not recommended to do so when you have a large number of records in a production environment. The first time the job runs, it will have to update all fields, which might lead to a performance impact.

Subsequently, the System Jobs option will have a Calculated Rollup field per entity that has rollup fields configured. By default, subsequent rollup field calculations will take place every hour, (which is the fastest schedule). If an hourly update is not necessary, you can update the value by double-clicking on the System Job option, and selecting Actions | Modify Recurrence. This will present a dialog box that helps you redefine the recurrence of the execution: