I have talked much on this topic before in here, so you can search some of my previous posts. For me personally, I change my routine up every 3-6 weeks on average if I had to guess. It all depends on the response I am getting from my body, the intensity level of the current program, the mental connection I am having with the routine, and a host of other variables.
Experimentation is the only way you are ever going to find what works for you personally. If I had to prescribe a set time frame for people to switch things up, I would guess 4-6 weeks. However, I don't think it is possible to accurately do this.
You will find people who will tell you they have had great results lifting the same way their entire lives. Then, you will talk to someone else who never trains the same way as he/she did yesterday in the gym.
Experimentation is the only way you are ever going to find what works for you personally. If I had to prescribe a set time frame for people to switch things up, I would guess 4-6 weeks. However, I don't think it is possible to accurately do this.
You will find people who will tell you they have had great results lifting the same way their entire lives. Then, you will talk to someone else who never trains the same way as he/she did yesterday in the gym.