I usually change my workout routine for myself, and those for my clients, every workout. However, as for a general answer to your question I would say when EITHER, your workouts are getting boring/stale (i.e. mental staleness), or when you are no longer making progress (physical staleness or plateauing).
I wrote an article about this very subject that was published in a tennis magazine. If interested, I’d be happy to send you a copy, just email me.
I hope this helps.