Daren,
Next to the Blue Winged Olive (BWO) the "Sulphur" Mayfly has a wide range of variations depending on time of day, month and location. There is the classic little yellow sulphur (Ephemeralla dorothea #18) and larger sulphurs (Ephemeralla invaria #16) that will take place on streams.
I have seen dun hatches on the Gun Powder, MD during the middle of the day and huge hatches on Penns Creek starting at 8:30 in the evening. Regarded by many as sulphurs by there similar yellowish tint, but I would suggest very different mayflies.
Sulphurs can even be lumped in with Light Cahills and Pale Evening Duns by many anglers. It can be really hard to tell to the untrained eye. Heck I have been fly fishing for 25 years and all I know is that I don't have it right!
The classic little yellow sulphur spinner is the the rusty spinner. My experience seems to be that the spinners return to the water in the evening. You will often see a mass of spinners moving from way up high around 7:00 and start hitting the water by 8:00-8:30. I can be a real frenzy we it all goes right.
It is best to know the stream and what to expect by experts in that area. Just as importantly, carry a good collection of spinners in a variety of colors and sizes. Yellows, greens, and rusty brown always seem to work not only for sulphurs, but as good general patterns. My favorites are #18 and #16, but don't embarrassed to have a deep field of #8 or #10. They don't match up with sulphur spinners but are darn fun to have when it gets dark, but that is a different topic.
Dave