The vast majority of dive lights do not have constant output. That means they dim as the battery runs down.
A couple of examples of lights that do have constant ouput are the canister lights from Underwater Light Dude, and the Dive Rite LX20+ and the Apeks Luna ADV.
If the manufacturer's info about a light does not say "constant output" then it's pretty safe to assume it gradually tapers off in brightness as the battery runs down.
Also, most of the manufacturers *ahem* greatly exaggerate the amount of light their light puts out. For example, the Orcatorch D710 claims to offer 1700 lumens on High and 3000 lumens on Turbo. There is NO WAY that light is putting out that many lumens. Not even when it is fully charged and first turned on.
If you see two lights with the same LED emitter, it is pretty safe to assume they will be about the same brightness, even if one manufacturer says 700 lumens and the other says 1700.
The standard LED emitter for years now has been the Cree XM-L2. They are generally good for around 1000-1100 lumens maximum raw output. If you look at the Cree specs sheet for it, you can find that it takes about (very roughly) 6 Watts of power to run that LED at (very roughly) 1000 lumens output.
With the now-standard 18650 battery at 3500mAh of capacity, some simple arithmetic will tell you that the battery is about (3.5 Amps * 3.7 Volts =) 13 Watt-Hours of capacity.
So, with a 6 Watt burn, a light with the Cree XM-L2 and a single 18650 battery could possibly output close to 1000 lumens continuously for about 2 hours.
In real life, those are approximations and there is some inefficiency, and it will actually be noticeably less.
Nowadays, there are some lights using newer LED tech (e.g. emitters from Luminus, instead of Cree) which are potentially more efficient at the same output level as the older Cree and also can go brighter while maintaining the same power efficiency.
Plus, 5000 mAh 21700 batteries are a lot more common now. So, you can now get lights that will burn brighter for the same amount of time, or burn the same brightness and last longer.
Regardless, the same type of simple math is easy to use to check many of these claims from the manufacturers, where you can determine for yourself that their claims must be false in some way.
For example: I recently got a Big Blue VTL9000P-Max light. They Big Blue rep explicitly told me that it would output 9000 lumens for just shy of 3 hours and then go into low power mode. I used time lapse photography with my camera in manual mode and found that it did not stay at 9000 lumens (or whatever its true max output is) for very long and after 2 hours it is a LOT dimmer.
The light holds 4 x 18650 batteries. Looking at spec sheets and doing some math, I verified that it would need roughly 3 times the battery capacity that it actually has in order to put out a constant 9000 lumens for 3 hours.
Many manufacturer claims are just straight up baloney.
Ones that I generally find to be reasonably accurate and believable are UWLD, Dive Rite, and Dive Gear Express.