Changing a workout routine is based on goals. If you are pleased with what you’ve achieved maintenance may be the answer. If you want to continue to grow and adapt then mix up the F.I.T.T. What is your goal endurance, strength, or power? The answer to that will lead you to which F.I.T.T. component to change. As a general rule adaption to weight training 3x’s a week is acheived after six weeks, 2x’s a week after 8 weeks.
Periodization routines can cycle every 3 months with mini cycles within the three months. See articles by Len Kravitz PhD IDEA expert writer.