As for duration: according to: Cam Specs & Effects, duration centerline separation and lift
Duration refers to how long a valve is opened in relation to crankshaft rotation. This open valve time period is expressed in degrees of crankshaft rotation. So, a cam specification of 220 degrees duration simply means the cam holds the valve open for 220 degrees of crankshaft rotation. (Is it ok to add that the duration of the cam is only 110 degrees since it spins at half the RPM of the crank?-Riley)
As strange as this may sounds, more duration can be helpful in high RPM engines but not low RPM engines. The extra degrees of open valve time in high RPM engines gives the air flow a little more time to get into (or out of) the cylinder in spite of the piston's stroke. However, at lower RPMs, more duration can cause less power because the valves will be open at the wrong time in relation to the piston's stroke up or down in the cylinder.
QUESTION:
As far as lift goes, is this measured on the cam itself, or are the rockers taken into account? So, for an advertised .500 lift cam, is the cam lobe height .500 from the core to the top of the lobe? Or, is it only .333 on the cam and when used with 1.5 rockers, it brings it up to .500? So if you added 1.65 rockers, the cam would give you .550 inches of lift. If lift includes the multiplication of the rockers, is it industry standard to use 1.5's?
