How many hours before you go to bed is it safe to exercise without it negatively affecting sleep?

A couple of hours: Generally your body needs an hour or two after exercise to achieve enough relaxation to attain sleep easily. Some of that relates to needing a small "meal" that includes protein and carbohydrates so that your muscles will build and recovery the best. The amount of time also depends on how vigorous and how long the exercise session was - the more intense, the more time needed before sleep.

2-3: Typically it takes 2-3 hours to wind down the body's chemistry to allow normal sleep.