The default busy handler callback is defined in main.c line 1646.
For systems supporting (and compiled) with support for sub-second sleeping (aka usleep and Windows) the times for the first polls are:
{ 1, 2, 5, 10, 15, 20, 25, 25, 25, 50, 50, 100 } ms
and 100 ms each thereafter. If the platform or compiler does not support (or has not indicated support) for sub-second sleep, then the frequency is once per second or whatever increment the platform supports (that is, if the platform only supports sleeping for a day-at-a-time then the polling frequency will be once per day).