I once tested my air consumption on dry land by just sitting on chair doing nothing and breathing from a scuba tank and measuring time + how many bars consumed from the tank. that came close to something like 10 L per minute (dry land, sitting doing nothing) .
when diving a person should be more relaxed I think but one probably will be swimming slowly... I tend to consume something like 15 L per minute or a little more when diving in very shallow 1-2m water but that will surely change when gaining more scuba experience.
One should test one's air consumption when diving and compare that to the exercise levels of the dive and dive profile (depth changes + time) to see how much really is consumed if doing, for example, a 40min dive with certain depth profile.
And then compare it every now and then to the new results to see if there is improvement and how big the difference is. Better buoyancy control should improve this a lot for example
