The unit you are likely going to try to work with is foot candle which is about 1 lumen/square foot (or there abouts). Lux if you want to deal with SI units and meters... but I'm on the left side of the Atlantic and so you're getting feet. Converting from fc to lux is left as an exercise to the reader. Full sunlight is about 10,000 fc. Overcast day is about 1000 fc. The moon is about 250 fc (that is the moon itself appears to be illuminated at 250 fc).
If you have a 500 lumen flashlight that spreads out over 100 square feet at the distance, that's 5 fc. If it only spreads out over 50 square feet, that's 10 fc. This is the calculation you are after. The beam will travel an unlimited distance, its just that it will spread out to the point where you can't see it.
If you are dealing with variable distances, you are going to have fun trying to redo the estimations for how long to paint each part of the scene.
At this point you figure out what the exposure you want on area in foot candles, say, you want it at 200 fc (and its the 50 square feet for 10 fc), that you are shooting at ISO 100 and need an exposure of 4 seconds for a 200 fc illumination (these are made up numbers - just to give an example of the type of calculation you will need to do), you need to shine the light on that area for 80 seconds (10 * 20 * 4).
However, this is only going to get you in the ballpark of what you are after, you will still need to do a bit of trial and error. This is because even tough its dark, there are many stops of light between one darkness and another. Light with no moon is a stop darker than light with a crescent moon, which is a stop darker than light under a half moon, which is a stop darker than light under a full moon, which is a stop darker than light under full moon with a snowscape. There are 13 more stops of light between the scene under the full moon and the exposure for the full moon (to the dismay of many people wondering why their moon photos are blown out).
The amount of contrast that you will desire depends on these factors. Too much light (and too little exposure on the rest of the scene) and what you are after is lost off one end or the other of the dynamic range.
Part of the point that I'm making here is that there are enough other variables in the 'how long to paint with light on a given area' that the amount of fudging for the lumen output isn't significant (or isn't significant to the orders of magnitude that photographers work with - 1.5x would be a big change for the manufacturer, its less than a stop for us).
For light painting, its a more a matter of 'get one that has the right color of light you are after (do you want LED 5500K light? or something more like an incandescent 2000K light?)' and then do some basic calibrations to get an idea of how much it spreads out at 10 and 80 feet away so that you can have an idea of how many foot candles you have at other distances so that you'll know if you want to light up a given area for 5 seconds, 5 minutes, or 50 minutes.
And then... try it. There is an amount of artistry that is necessary in light painting that can't be pinned down with numbers.
