When I soak oak in wine or liquor, I soak in just enough to cover. As far as how much oak to use, your oak probably has directions on the package, (it's probably 1 pack or 1/2 pack per 5 or 6 gallons). And I use just enough wine/liquor to cover the oak. It's up to you how much oak character, and residual wine/liquor character that you want. I usually use spirals, not cubes, and according to the package flavor is fully extracted in 6 weeks- your cubes may have a different timeframe. Basically anything after that won't add additional oak character. I'm usually going for mild oak and more of the of residual wine/liquor character, so I usually soak the spirals as soon as the beer is in primary, and then after 4 weeks primary or so I'll remove just the oak and add to the beer in secondary, leaving the soaking liquid behind (with most of the oak character extracted already). If you want more oak/less wine/spirit you could soak them much less time. Or soak them prolonged and then add some of the soaking liquid too. Or add some of the spirit with the oak.
So there's a lot of ways to go, and it depends on what you want.
My Brett Old Ale is aging with Tawny Port oak. Used a medium toast French oak, two spirals. Soaked for 4 weeks, and then added the oak and maybe a 3rd of the residual port in secondary. Now, I don't know if that's a perfect fit as I haven't tasted it since (it's only been a few months since). But that's what I did at least.
