Constant RTC_READ_TIMEOUT_MS 

pub const RTC_READ_TIMEOUT_MS: u32 = 10_000;

How long rtc_read_once waits for a dated sentence before giving up, in milliseconds.

A receiver whose backup domain stayed powered emits a dated RMC within a second or two of its main domain coming up — it is reading its own clock, not searching for satellites. Ten seconds is generous enough to cover a slow start and short enough that a receiver which has lost its clock does not hold up a boot.