If I have hemophilia, when does infusion become necessary?

Depends on type. There are different types of hemophilia, and the need for infusions varies by both the type of hemophilia you have and the type of injury sustained. If you have a severe clotting deficiency, you may need infusions with even minor bleeding episodes or injuries. If you have a mild form of hemophilia, infusions may only be needed with major trauma or major surgery. Talk with your hematologist!
Depends. It depends on blood levels and bleeding. Severe bleeding or injuries require earlier treatment. A hematologist can assist with this.