Joe, when I test photocells, it typically takes about three minutes for the lights to come on, after I have covered the sensor with black tape.
When I first apply power to the photocell, it takes about half a minute for the photocell to activate the lights - and another minute or two to shut off the lights if it's sunny out.
If a photocell is being fooled by reflected light, the light can cycle fairly quickly; any HID light will take a bit to come up to full brightness and trip the sensor. By "quickly" I mean maybe 15 seconds on followed by three minutes off. Reflected light will not cause the extremely fast cycling described by the OP.
