It really depends on how much flavor you want to impart, oak is a strong dense wood so it does not absorb ready, a friend of mine is doing the same thing right now and soaking for 6 months prior to use. Additionally then the final product will depend on how long you oak in the secondary. I made an Oaked IPA- not to style BTW, and in the secondary for 1 month, imparted a nice gentle bourbon flavor to the beer, not overpowering and subtle aroma but my next batch will be a longer soak and longer secondary, also higher ABV and a little more hop to balance it all out.