It really depends how quick your body responds. For me hitting a platue can take 4 months, but I work out different body parts and change almost every week. If your hitting the same routine every week, then I say no later than 3 months but only if you have reached your maximum potential from that exercise for the muscle group.
1. No visible changes 2. regression of strength & Stamina 3. increase in body fat composition by using an Omron or other electric devise to measure lean muscle mass. Muscles have a memory and it if your routine remains the same without a "shock" it expects and adapts not allowing progression.
