Progress is the real signal
A routine works while the numbers climb. If you added reps or load last month, keep going. Program hopping every two weeks resets your momentum before adaptation happens.
Change your routine when progress stalls for two to three weeks despite good recovery, or when your schedule or goal shifts.
You do not need a new routine every few weeks. Keep one as long as you are still adding reps or weight and staying consistent. Swap it when you have genuinely stalled on most lifts for two to three weeks with good sleep and food, when you get bored enough to skip sessions, or when your available days or goal changes.
A routine works while the numbers climb. If you added reps or load last month, keep going. Program hopping every two weeks resets your momentum before adaptation happens.
One rough session is not a plateau. Look for two to three weeks of no progress across several lifts, with sleep and nutrition in check, before you rebuild the plan.
Dropping from four to three days, shifting from muscle gain to strength, or nagging joints from an exercise all justify a change regardless of how long you have run the plan.
Change your routine when progress stalls for two to three weeks despite good recovery, or when your schedule or goal shifts.