+1, I use at least a little bit of oak in every mead, even the ones where I don't want any actual oak character. Just a few days with the oak will "round off" the alcohols and other sharp flavors, and provide just a slight bit of tannin to help offset the sweetness.
As to how long, you will have to be the judge of that. If I'm planning on imparting an observable oak character, I would start taking daily samples after the third or fourth day... the last thing you want is too much!