At night, the beam of a >15mW laser or even a 5mW green laser will be visible to the person on the end of it. Even if it is dimly lighted outside (late dusk) they may still be able to see a glare from the laser even if they cannot see the beam.
However, how far the beam will travel and how far away you will be able to project a dot are different.
The typical divergence of a green laser is around 1.2mRad (milliradians). This means that the beam will expand about 1.2mm for every meter it travels.
For example: A laser with an initial beam diameter of 1mm at the aperture and a divergence of 1.2mRad would project a dot roughly 1.2 meters wide at 1km (1 kilometer = 0.6 miles = 3168 feet).
Lower divergence is ideal, however it is rare to find a laser with less than 1.0mRad divergence without additional optics. The divergence can be decreased by the manufacturer or even yourself, but that's a different topic.
With a spot 1.2 meters wide at 15-25mW, the dot may not be visible at that distance.
However, even with a beam about 1 meter across, the light will be noticeable if the observer is in the path of the beam. 15mW of green is actually pretty bright.
Another characteristic of lasers is that the beam is noticeably brighter when looking toward the source than it is when looking from or perpendicular to the source. This is because of diffraction of the laser beam off of particles in the air. This diffraction is more noticable when looking toward the source because a larger portion of the light will be diffracted in the direction the beam is traveling. The beam is least visible when viewed at a right angle because the fewest photons will be diffracted in a perpendicular direction.
For more information on this, look here:
http://en.wikipedia.org/wiki/Raleigh_scattering
I hope this helps to answer your question. Sorry for the length, but many aspects of lasers cannot be explained in just a couple sentences or even with a simple yes or no.
Have fun reading here and welcome
