Doing the same thing for forever doesn’t produce results due to the Law of Diminishing Returns. This can be circumvented to some extent by some macroscopic fitness programming. It’s not a routine, it’s a routine about routines. I’ve heard recommendations about changing programs every 12 weeks, and another that suggests changing every month.
This all mostly seems to apply to strength training, specifically.
Example: Huberman’s Routine
A - Heavier weights, 3-4 sets of 4-8 reps, longer rests (2-4 min) B - Lower weights, 2-3 sets of 8-12 reps, shorter rests (60-90 secs)