Found the document of House Rules about Speaker Pro Tempore
https://www.govinfo.gov/content/pkg/GPO-HPRACTICE-108/pdf/GPO-HPRACTICE-108-35.pdf
§ 6. The Speaker Pro Tempore
Appointment or Election
The Speaker may appoint a Speaker pro tempore. Such an appointment
may not exceed three legislative days, except that in the case of illness the
Speaker’s appointment may extend to 10 days with the approval of the
House. Rule I clause 8. For longer periods, a Speaker pro tempore is elected
by the House.