I am interested in throwing some in the secondary as advised in radical Brewing, but would love to here what has worked and failed from other brewers. I was thinking around two quarts. I want subtle flavor and no beer bombs!
I haven't used molasses, but I've used black treacle. It's like molasses, but darker. Molasses has a strong flavor. I think two quarts would be an awful lot. When I used treacle, I used about 4 ounces by weight, IIRC, during the boil and the flavor was there. I'd add it during the boil to help ensure the sugars get broken down and the yeast can eat them, and I wouldn't use much more than a cup or cup and a half of it. It provides much more flavor than, say, maple syrup.
I had looked into molasses a few batches back. I was wanting to impart a slight rummy flavor to my beer as well. The problem I've read (I didn't actually end up using any) is that too much just makes your beer taste like really bad, cheap, undistilled rum. That being said, I would err on the side of caution and use less as JackSmith suggested.
I haven't used molasses but, I've used maple syrup. I added it at the end of the boil. I added 14 oz to a six gallon batch. I'ts pretty good. I think that using two quarts might be a little much depending on how big you're batch is. Also your going to sweeten the batch up a lot if you're not careful with your ratios. I would try a little to begin with and then adjust your recipe. Don't add the molasses in at the begining of the boil. It can make your beer have a cidery flavor.