I don't know really. Depends on what it is. If it's like work or something, or doing something productive and fun, it can last hours, or days even
But NORMAL every day stuff such as having a conversation, eating, umm and other things, it's really really short. I don't pay attention and I just change the subject, or talk about 5 different things at once
I'm usually quite a slow thinker as well. When I'm out shopping, with my intent on buying a pair of tights, my mind goes off it and I end up buying bags, tops, jeans, ANYTHING but that =) That's why you need a good companion or something with a great memory to stick with you and constantly remind you / poke you.