An aero helmet is generally 30-60 sec over 40k. Make sure to get a nice one, though. The ones that look like they're about halfway between a full-fledged aero helmet and a normal road helmet don't do as much. Get an LAS Chrono, LG Rocket (not an LG Chrono), Bell Meteor, Giro Advantage, etc. The differences between them are very minimal and will differ from person to person, but can really only be optimized in a wind-tunnel, which very few people have access to, so any of them will give almost identical results. IMO, though, stay away from Rudy Project. A bit less aero than the others. My roommate had one for a year, and is getting rid of it in favor of one of the others.

I notice a difference in the force on my neck when I put on my normal helmet, but other than that I don't have any empirical evidence of how much faster one is at specific speeds. I now race and train in my LG Chrono. It has the effect of really cheesing off traditional bicyclists (esp. the guys in wool shirts), who then feel the need to try and race me on weekend rides to show me how much of a fool I am, I suppose....but up to this point I'm undefeated, so who's the fool now!!!!! ;)

I've never seen a wind-tunnel analysis comparing different brands of helmets, but the way that aero helmets work is by reducing flow separation behind the head. When eddies are formed behind you, they create a low-pressure zone (drag force) that slows you down. Having the tapered fairing there in the place where the eddies would be reduces that separation. Rudy Project helmets just don't come down as far, leaving a larger gap above the shoulder blades for flow separation to occur. Wearing a Rudy Project helmet vs. a road helmet isn't going to slow you down or anything, it's just not going to be as fast as the others.