Constant DEFAULT_APP_DATA_CALLBACK_TIMEOUT 

pub const DEFAULT_APP_DATA_CALLBACK_TIMEOUT: Duration;

How long a single app_data callback may run before it is abandoned.

Sized against the round trip the callback exists to perform: merge, then publish the result with update_app_data, which commits, publishes, and waits for the intent to resolve — a few seconds on a poor mobile network. Ten leaves room for that while keeping the worst case a host can inflict on its own sync() call short enough to sit behind a spinner. Raising it buys slow networks more headroom at the cost of a longer visible stall; the balance is why this is a field rather than a hard-coded constant.