At Kogalla, we make some really awesome and very bright trail lights. They put out 800 lumens continuously. When I tell people this, I often get asked, 'How bright is a lumen?' Usually, I turn on the light at full brightness, trying not to shine it directly into their face, and say "This is 800 lumens, so imagine this but only 1/800th as bright and that is one lumen." That answer is precise, but not all that helpful. When a child asks 'How long is a foot?', you typically hold up your hands about twelve inches apart and say, "About that long." That answer isn't very precise, but is quite helpful. So to help myself and others get a more intuitive sense of how much light a lumen is, I'm going to compare different types of light sources, and their associated lumen output.
The first one I'll start out with is the sun. From the perspective of a biped, humanoid life form living on a relatively obscure planet near the edge of the Milky Way galaxy, the Sun is the brightest light we experience. Of course, the known universe is home to a virtually infinite number of heavenly fusion reactors that are many times brighter than our own sun. However, their distance from us is so great that knowing details about them regarding electromagnetic or light output doesn't help our intuitive understanding of how much light is one lumen.