The short answer is that it depends.
First of all, "background refresh" for the app must be turned on in the system settings.
Apple gives little insight into how much time the app are allowed to run in the background before it is scheduled to execute less frequently or is suspended. This seems to vary across iOS versions.
It also very much depends on what device you are using to sync with your Wave, Wave Mini, or Wave Plus. Android or iOS? Android 9 or 10? Samsung Note or Galaxy? The Bluetooth capabilities are dependent on multiple factors.
However, we currently see 50% of all data sent in to us is send in from "the background" on iOS vs 25% on Android.
This is something that we are trying to improve on, so we are working to improve the total time a sync takes and also wake up the app from our cloud periodically to start a background sync process.
If you are interested in this topic, you should know about our new ethernet-connected Hub product coming out in 2020!
It will bring all of your devices* online, and erase the need to sync via Bluetooth. Data will always be available from anywhere with down to 5 minutes sampling intervals.
*The Hub is compatible with the Wave 2nd gen, Wave Mini and Wave Plus