At what point should you change the exercises in your workout routine (weeks, months etc)?

Wow, you have had as many opinions as you have netted responses. 😀
Generally, if an exercise isn’t providing a training effect any longer or you’ve plateaued, switch to another exercise. Constantly stressing the bones, tendons, ligaments, and joints with the same activities and in the same way day in/day out leads to stalemating longterm. Manipulate lever length, grip, rhythm, tempo, direction, body positioning, and ROM to shock the muscles into continued growth as well as to provide more even wear and tear on the joints instead of always stressing them in exactly the same manner & with all the same exercises. This also keeps boredom at bay and keeps things fresh and different.