I use 2 ounces of medium toast American oak cubes
per 6 gallon batch of cider. Typically, I'll infuse for 3 months before racking off. Oak and cider go together very well indeed. Keep in mind that my ciders and apfelweins follow the following fermentation / aging sequence:
2 months primary
3 months secondary (with oak)
7 months - MINIMUM - bulk-aged before kegging and / or bottling.
Most ciders are drank way too early IMHO - let a batch bulk-age for a year or two and you'll know what I'm talking about.