I prefer to use oak cubes, as the size is more consistent than chips, so I can better decide the amount to use. Using medium toast cubes, I will use as much as 2 oz in 6 gallons of wine. However, since I prefer oak as a seasoning, not a flavoring, I typically use 1 to 1-1/2 oz, depending on how heavy the wine is. Heavier wines can accept more oak without being over powered.
Once I add oak, I leave it in the wine. My current take is that putting oak in for short periods and removing makes the wine sharper. Leaving it in the wine for 3 to 12 months seems to mellow out the flavor.
Depending on how small the chips are, chips have more surface area than cubes and will affect the wine more. For that reason, if using chips, I'd use 1/4 oz less than the above amounts. Go with 1-1/4 oz per batch and gently stir the wine monthly. Wine does not have convection currents so the wine next to the oak will be much more heavily flavored. Stirring will homogenize the wine.
Keep in mind that you can always add more oak, but once the flavor is in their, it's quite difficult to remove. So ... go light at first.