I've used the Event Administration feature to chose the data sources from what is configured in Contact Builder, define how often to inject new subscribers, and what criteria a subscriber has to meet to enter the Journey.
You can actually schedule the periodicity as shown here.
Use this step to determine when and how often Journey Builder checks
the event source for new records to inject into the Interaction.
If contacts are injected into the Interaction via API or using
Automation Studio, select the top option. Setting an entry schedule
here is not necessary. If contacts should be injected according to a
schedule, select the bottom option.
Step 5: Set Entry Schedule
Set the date, time zone, and repetition schedule. The default Repeat
setting is None, which means the entry event runs only on the
scheduled date and time.
The Start Date set here is used by the entry event that is being
configured. This schedule dictates when Journey Builder checks the
event source for new records. The schedule set here does not inject
contacts into the Interaction unless the Interaction has been
In cases where the schedule set here begins prior to the Interaction's
activation, Journey Builder only injects contacts that are newly added
to the event source after Interaction activation.