I personally change my training TYPE/STYLE completely every 3-4 months. I find that even with proper planning after that long on a type of training you get accommodated and progress stops.
Here is a little something from an article I'm working on:
Accommodation: The process where your body becomes more and more adapted to the training you are performing. The more accommodated you are, the lower your trainability is.
Trainability: This represents the potential for improvement of a muscle, movement pattern or physical capacities. In simple words the more trainability you have, the more you can progress. Having a higher trainability normally also means that you can progress at a faster rate. It can be affected by genetic elements (some people have more potential for muscle growth than others for example, so their muscle trainability is higher), testosterone and IGF-1 levels (the higher they are, naturally or not, the higher is your trainability), experience (the more you have trained an progressed in your life, the harder it is to progress because your trainability is lower), etc.
Accommodation is what happens when your body is pretty much fully adapted to a certain type of training; you are adapted to the “means” (exercises, special tools like bands/chains, barbells vs. dumbbells vs. pulley, etc.) to the “methods” (how you are performing the exercises) and the “strategies” (intensity level, order of exercises, workout density, etc.).
It is a normal thing and something we should work toward. Understand that when you are training, your goal is to stimulate changes in your body (adaptations). When your body is “accommodated” it means that you did an amazing job at stimulating adaptations. If you never work toward accommodation, you aren’t going to stimulate any gains.
The problem is that the closer you get to being accommodated, the less trainability you have. Trainability is how much adaptations, or progress you can stimulate. The more trainability you have (or a muscle/group of muscles have) the less room for progress there is and the slower that progress will be. Once you are accommodated it is almost impossible to stimulate further gains. In fact if you have been in an accommodated state for a fairly long time you will start to lose some capacities, increase the risk of injuries and will lose motivation.*****
If you change "program" but you stay with the same type of training you can circumvent accommodation for a while. But at one point, changing the exercises and even loading parameters (as long as you stay with the same style) will not prevent accommodation anymore and you need to change the style/type of stimulus completely.
And the more experienced you are, the more often you need to change. I personally prefer to keep the exercises similar but change the methods, means and style of training.
That's why sometimes my own training my seem all over the place: because if I stick to the same TYPE of training for too long I stop progressing an actually start to regress (signs of being fully accommodated).
The article will cover the whole topic of variation in much more depth