You want to oak once the wine stops dropping lees, and it's ready for bulk aging. I normally age on oak for about 30-45 days if I'm using oak chips, but longer with oak cubes or spirals. I really like Hungarian oak for strong but rich oak flavor, but American oak is good if you don't over do it. They come in several different varieties as well as different "toast" levels. I think medium would be a good place to start.
I've oaked concord wine, but you have to use a very light hand as it's very easy to overdo. It's not very deep, and don't hold up to much tannin from the oak.
I currently have a beta cross (native grape) that turned out far less tart and thin than concord wine, and I'm going to oak that on the next racking.
Concord wine tends to finish very dry, and so it can be very very tart. Sometimes people prefer it sweetened a bit, rather than oaked.
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