For me, the injection time of a part is decided based on four mains areas:
1. Material (which is related to its viscocity and flow length)
2. Size and general thickness of the part (which is related to viscocity and flow length of the material)
3. Pressure (should be below 70 MPa if runner is ignored)
4. Problem of short shot (due to complexity of the part and freeze-off problem)