are you asking how many times it can be used till it loses the flavor? Barrel can be used to make great sour beers after they lose their flavor. the microbes live in the wood and when you pour the wort in there you don't have to pitch anything and the microbes live on.
I'll drink a sour beer now and then, but I don't want to brew 50+ gallons of it. I'm wondering if the oak will contribute anything to a beer after it has aged bourbon and a mead.