Most of the time I take about 5-10 minutes walking around looking at things. I never take a framing device of any kind...I just kinda' know what will show up on the ground glass given the lenses I have. I also have a pretty good sense of how a scene might change given different light. If I scout a location for more than 15 minutes, I'll usually give up on it if I haven't found something. Whether or not I have my equipment with me depends on whether I'm shooting 4x5 (when it's always strapped to my back) or 8x10 (for which I have yet to find the right backpack).
Once I've selected the location, if the light is right it rarely takes me more than 10 minutes to set up, shoot, tear down, and move on. If the light isn't right but I think it might be better at another time, I put it into my mental checklist of things to look at again when the opportunity arises and I move on.
One of the reasons I love living where I do is that, between the hills and valleys and the lattitude, we not only have four seasons but we also have some fairly predictable light and other features. If I know a scene would look better in the fog, I know about when I'll have to schedule a trip back. The same with bright sun, snow, rain, overcast, or anything else short of frogs falling from the sky.