If I were to pour 12 to 16 ounces from the bottle, how long would the beer left in the bottle stay carbonated?
depends on what you mean. It will start loosing carbonation instantly. if you’re talking about going from 2 volumes to 1 volume (i.e. losing the appropriate carbonation for the style) thats one thing, going still is another. Keeping the beer cold will slow down the process since cold beer holds more CO2. Generally for my 64oz growler, the beer has lost a little but is still adequatly carbonated the next day
I’m just refering to wanting to drink a beer without drinking the whole 32 ounces at one time. If I put the rest back in the fridge, will the carbonation last for a little while without going flat? Or, do I need to drink all 32 ounces within a certain amout of time to avoid having it go flat?
Depends on how many times you open it. If you open it once, pour a glass and then close it, then open it the second and last time for the second glass you may not notice a whole heck of difference.
The best way to determine for your tastes is to try it for yourself. The more times you open it, the more CO2 will be lost, and the more exposure to oxygen it will receive.
A day or two will have a noticeable difference, but to most people, it will still be very drinkable. Definitely keep it cold after resealing though.
It won’t be quite correct due to the increase in solubility from head pressure, but you can just take a weighted average of the difference between the volumes and subtract it from the difference between the initial and saturation pressures, and get really close. So for example, if you have 32 fl oz of beer at 2.4 vol and 41°F (~1.4 vol saturation) and pour out 12 fl oz, the remaining 20 fl oz will have:
1.4 + (20/32)(2.4 - 1.4) ~= 2.0 vol