Oak barrels

That's pretty arbitrary information. The fact is that the possibility of infection is there from day one. There is no certain number of batches before infection.

You can mitigate infection through proper sanitation.

 

Never done anything with a barrel, so ...

I think most people end up using barrels for sour beers after 2 or 3 clean beers because the whiskey (or whatever) has mostly faded at that point. I don't think that bacteria is spontaneously generating in used barrels after a couple batches, you have to add the bugs that you want.

@FATC1TY has done some barrels, hopefully he can set you straight

 

If your sanitation is up. You keep the barrel clean and hydrated. You can get a couple beers easy. I've gotten 3-4 and at that point I soured it myself.

The oak flavor or spirit in the barrel beforehand changes nicely and it's better to go dark and heavy to lighter each pass I've found.

I'd plan for atleast 3 or more beers in a barrel if you are a really attentive brewer and cellar man.

 

I have a 5 gallon barrel that I've been using for the last 3 years. I think total I've put 8 batches in it and in fact I just refilled it yesterday. The first two batches were "clean" beers with no problems, then I intentionally soured it. As long as you keep it full it should last you for a while. As far as continuing to produce clean beers, its lifespan is going to be shorter. There is a much greater risk of an infection holding in the barrel. If you get an infected batch in a carboy, you can sanitize and it should be fine for the next batch, but a barrel will not work that way. So for clean beers I would say generally 2-3 beers, but after each batch the barrel character will be weaker.

 

By hydrated do you refer to spraying down the outside of the barrel while the beer is aging or simply keeping them full of beer?

 

Both. Once filled it's enough though