How often should I change my workout routine for great results?

I was thinking of revamping my workouts every two weeks. Is that too soon? I know you have to keep the body challenged so you don’t plateau.
For example, for almost two weeks now I’m strictly doing:
biking 3x a week
walking 3x a week
ab workout 3x a week
yoga/or just making sure I have a good stretch session 3x a week
upper body work w/ weights 2x a week

Then in my next two weeks I was thinking of these workouts while keeping certain things consistant:
biking 3x a week
walking 3x a week
lower body work w/ weights 1x a week
yoga/stretching 3x a week
full body work 1x a week
upper body work w/ weights 1x a week

And, maybe the following two weeks, I’d focus more on Pilates, stretching and cardio.

All this with having one or two rest days a week of course.

Suggestions?

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