A quick google finds a few threads elsewhere, some of which have calculations listed to work out the difference in volume based on temperature etc, but short answer, there are a lot of variables including fermenter size, shape, volume, headspace... there are 'co2 harvesters' out there (essentially two mason jars connected with tubing, purged with co2 during ferment, then sucking back the co2 after the fact. Of course there's potential the co2 mixes with the o2 in the jars, so may not be fully purged via this method (and as you suggest, after a certain point may bring in o2).
Some people purge a keg and hook this up, I've been using a couple of PET bottles with carbonation caps / tee piece - fill with sani, push out with co2 (to try and remove as much o2 as possible from the get go) then add a few psi of co2. Insert tubing into the airlock / bung hole to plug, attach disconnect to PET bottle and this allows a slight reservoir (and some co2 in the headspace). Been doing this with a brewbucket which can hold a couple of psi. It definitely pulls in co2 and some absorbs into solution, so I do re pressurise the PET bottles a couple of times over the few days cold crashing.
Mylar balloons also work great, and can hold more co2, and a simple visual indicator (I have limited fridge space hence going for a more compact setup).