>5mw can damage your retina if it directly hits your eye. Anything <5mw is thought of as harmless because your blink reflex will usually save you from any damage.
Here are some general guidelines.
1-You should always use goggles when burning stuff with powerful lasers.
2-Using 5-50mw lasers without goggles can be done if you are extremely careful and watch out for reflective objects. Don't look at the dot on close walls/brightly colored objects either.
3-Always use goggles when using a >100mw laser inside. All it takes is a bad reflection to do instant eye damage at that power level and you can hurt your eyes by looking at the dot on walls.