The longer I write music, the more I think in terms of timbre minimum.
Timbre, to my ears, is more important than specific voicing. What kind of sound is it? Is it unison? 2-part? 4-part close position? 4-part open position? 6-part? Is it vocal percussion, vocal bass, and three part harmony singing pentatonic lines? All are great sounds to use.
A good rule of thumb I’ve found is a minimum of 4 measures between timbral changes. You are trying to find the balance between surprise and familiarity: one on side, a single timbre that last so long that our ears get bored; on the other side, so many timbral changes that our ears get confused.
It’s my experience that young arrangers (and I remember falling into this camp) are more likely to change timbres too often than too seldom. We have just discovered these various sounds, and want to use them. But change them too quickly and you get a muddled sound that won’t read for the audience or sing for the musicians.
Of course, it’s probably better to think in real time. 4 measures at a medium swing tempo of 108 will last just less than 7 seconds in real time. 5 to 10 seconds between timbral shifts is a worthwhile goal, but musical phrases should dictate more than precise temporal rules.