Less is more when it comes to unused oak chips in beer, IMO. Decide on the amount you think it right, then half or quarter that amount to actually use. Decide on the amount of time you think it right, then half that length and start sampling.
If it were me, 2.5 gallons of a Mild ale, I would opt for no more than half ounce and only give it a few days before sampling. If you haven't reached your desired level of oak within a week then consider adding a small amount more (eighth or quarter ounce maybe).