It all depends on your clients adaptations! There are many adaptations you should consider before sequencing and progression to new exercises. An understanding of neuromuscular adaptations, physiological adaptations are important to determine WHEN to advance. Movement patterns should be looked at to determine speed, range of motion and posture before continuing to a new exercise. If any sequence in the chain is flawed when perfoming a movement pattern/exercise you should not advance until a client has gained the neuromuscular ability to memorize the pattern. (According to research this can be usually around 2-8 weeks for the onset of the neuromuscular chain to cordinate the exercises/movement patterns. Late phase adaptations are including muscle fiber hypertrophy and can be seen usually after 16 workouts according to research.) However these are not definite numbers but should be kept in mind when sequencing and progression.