I happen to like Baroque-style ostinato patterns (mostly broken chords) for use in my compositions, but I'm always a bit unsure when is an opportune moment to break the pattern and change into something new. I usually judge the breaking-off point by the whim of my ear nowadays, i.e., I change whenever I'm starting to feel bored with the pattern.
Is there a more rational way to approach this (like the golden ratio or something), or does it all depend on the composer's innate sense of proportion and/or tolerance of boredom?