How to Avoid Overtraining
Periodization is the planning of a workout program that starts in phase 1 to target the endurance fibres in the muscles, with lots of reps and a low load. Gradually over several weeks, the load is increased, but the volume (sets x reps) is decreased. In the transitional phases, the program switches from targeting the endurance fibres to the strength fibres. Rest and recovery between sessions are absolutely essential.
A trainer should be planning this transitional programming, with about two weeks in between changes in load or volume. It takes that long to adapt and get prepared for changes If you are having problems with excessive muscle soreness that persists longer than a day or two, mention it to the trainer and some regression is in order. When forward changes do happen, the trainer should limit the stress to about 5% increase.