This depends on the device and driver. I'm also wondering the above, what make and model is this device?
At least a few seconds for one device, but longer doesn't hurt (you could go minutes or even longer--it doesn't really matter if your threshold isn't lower than that). Doing this on only devices that might "need" it is indeed likely better, as it reduces traffic on your network and using more power from devices that don't need to do it.
But keep in mind that not all battery-powered devices will respond to a refresh. In my experience, most major-name Zigbee products do (CentraLite, SmartThings-branded products, Iris-branded products, etc.), whereas some cheaper brands don't (Ikea, Sonoff, etc.), but this will likely depend on the device and possibly driver. You only mentioned Zigbee, but for Z-Wave, almost none will unless they are "FLiRS" devices like locks, blinds, or similar devices that need to respond to commands from the hub while being battery powered.
I did write an app to make this easier, including refreshing: [RELEASE] Device Activity Check - Get notifications for "inactive" devices. However, it still won't help you if your devices don't "report in" on a regular interval; the problem with any approach is that you need to know what's normal for your devices in terms of Last Activity At data, and many devices don't have reliable/predictable behavior in this regard.
But it's still probably better than battery level. 