Well, if you’re training for strength any hypertrophy then you’d probably want to keep the set duration between 10-40 seconds (reps will vary with the exercise chosen). But if you were trying to recreate race specific energy systems demands then you would likely do one rep per step in the race.
And the ROM over which one trains is important too. The hip flexors do their work when the femur is in line with the torso (hip neither extended nor flexed).