The amount of hz that the Fs will drop depends on the woofer itself, every woofer has different changes in T/S parameters.
And there isn't really a true break in period, the spider will keep getting looser and looser, so Fs will keep dropping till the spider rips.
And how long you need to 'break in" depends on the user + the driver itself so giving a estimate is impossible.
Fs is just the resonant freqeuncy of the driver, like a sheet of metal has a resonant freqeuncy, the 'ting' you hear when you hit a sheet of metal.