Everything they said ^^^^, plus, I use 5 gallon barrels for wine, mead, cider, and beer. Because the oak/spirit character is going to change over time and with every batch, I never know how long my stuff is staying in the barrel. The port stayed in barrel for half a year, the red wine I'm getting ready to move after two months. The beers? Sometimes as short as a week or two.
To avoid opening the extended-aging barrels too often to top up, I limit the amount of O2 transfer and evaporation by sealing all sides of my barrels with wax. If I was using larger barrels, I would probably seal less surface area. Additionally, with wine, I always have some leftover after filling a barrel for topping up.
The barrel with red wine is going to get a sour beer soon, so then it will become something of a solera-style; where I bottle maybe 4 gallons out of it 1-2 times per year and then top up with fresh wort. For now, though, it's acquiring some serious red wine character to go with the oak.
Below, my assistant helping me wax barrels and filling up one from the fermenter.