At night, you can see 15mW even, but in absolutely regular lighting conditions with no fog and such, you would need hundreds of mW. My 300mW has extremely good divergence, and you cant see it in daylight without an aid, or looking at it from a good angle. However with a fogger, you can see even <1mW, and at night, >10mW should be visible.
my 35mw is visible in standard 75watt lighting
i wouldn't say it's totally obvious, you have to look for the beam
it's fairly faint until you shine it into a darker area of the room, also depends on humidity and dust in the air
it's more visible with lithiums also