A megasecond is a useful quantity: it's a bit less than 12 days. I mentally approximate it to a fortnight, partly because of the VMS bootloader which used microfortnights as an approximation for seconds https://en.wikipedia.org/wiki/List_of_unusual_units_of_measu...

Another handy time_t approximation is that a year is about 2^25 seconds, or 32 megaseconds.