RE: How dark is to dark to shoot? what range of sight?
In most areas, legal shooting light is from 30 minutes before sunrise until 30 minutes after sunset. If you are in flat terrain, and there are no clouds, you can just note when the sun dissapears and shoot for the next 30 minutes. In rolling terrain, this doesn't work and of course, it doesn't work in the morning.
Since sunrise/sunset is changes every day, the easiest thing to do is to check a local paper in the area you are hunting for sunrise/sunset times and go by that. There are also links on line that will give you these times for any locality. In reality, you can most likely be legal if you can still see the deer clearly with your naked eye. If you can't make him out clearly (not only is it a deer, but is it a buck or doe) without the help of a scope, it is probably too early/late.
The shots you heard were most likely before/after legal shooting time, so I wouldn't use them as a guideline.