How important is it to clean your drivetrain? For the first time in my life I took the time to properly clean the drivetrain of a bike. How proper it was might be debatable, but for the first time I saw that the gears are not actually black.

I've owned bikes since I was a kid, when I grew out of one my parents bought me another one. I never regularly commuted by bike, but at times I was cycling quite a lot. I never though of cleaning my drivetrain and I never noticed any problems with it. I do remember lubricating it, but I'm pretty sure we didn't clean it before and we certainly didn't degrease it.

After I started cycling as my main mode of transportation, I also started reading and watching videos about bike maintenance and cycling in general. Some people seem to point out the importance of a clean drivetrain whenever they get a chance, whereas others don't seem too bothered. Cleaning your drivetrain is probably one of the cheapest things you can do to improve your bike's efficiency, so in a racing situation, I don't see why you wouldn't, but for normal commuting it doesn't seem that obvious.

For most people, the difference in efficiency isn't important. They are more interested in spending as little time and money as possible on maintaining their bike. Not cleaning your drivetrain will definitely save you time, but will it also save you money? 

I understand that there are a lot of variables to consider here, but the main one is how much faster your chain and sprockets will wear out. Things like price of components, cleaning products, and time spent are also important, but much easier to research and estimate. I know quite a few commuters that never clean their drivetrains, for some, even keeping good tyre pressure is too much of a chore, and they are riding the same bike, with the same drivetrain, year after year. 

On more expensive bikes with more expensive drivetrains, it's obviously more important, because the components are more expensive to replace, but is there a point where your drivetrain is so cheap to replace that you might as well not bother with the maintenance or just do the bare minimum? Like, could you get away with just lubricating the chain, without cleaning it?

When you google the subject, you find results that say that your drivetrain will last much longer if you keep it clean and lubricated, but I haven't seen anyone quantifying it. Much longer could mean anything. Is it 10%, 20% 50%, 100%, 200% or 1000% longer? Also, how much longer will your components last if you clean your drivetrain once a week, compared to once a month, or once a year?

I don't mind you sharing your opinions and personal preferences, but please do back them up with empirical data and maths. I'd like to see some proof that my efforts are worthwhile, beyond the fact that I like the feeling of riding with a clean drivetrain. Ultimately, I would like to be able to make an informed decision about how much effort I spend on bike maintenance.
I used to work on a fleet of cyclists' bikes who used their bikes as part of their full-time job, where it was usually raining and dirty. I changed a lot of drivetrains and oiled a lot of chains over the years. That said, I don't have any kind of empirical data to say that doing x will make your chain last y% longer. But I can speak generally about the relative factors at play, and what is worth considering when you're looking at your own personal bike / fleet and trying to decide where to invest your time and money. 

First, it makes a huge difference who is riding the bike and how they ride. If they are a bigger person who likes to mash the pedals, they will burn through drivetrains much faster than a smaller person who tends to spin. Some people, the 200lb+, aggressive riders, needed new drivetrains every 2 months. A smaller, more cautious rider who tends to spin might take a year or more to wear out the same chain. 

Another big factor is what kind of lube you use, and what conditions the drivetrain actually experiences. The ultimate cause of the chain wearing out is the holes for the pins will wear out as the pins rotate under tension and sand or dirt works in, effectively acting as sandpaper along the inside of those holes. How you oil the chain, and what kind of oil you use, also plays a big role in how much dirt and grime get picked up. Personally, I think a wax lube is really good at keeping grime and sand out, and makes my chain last longer while needing less cleaning. I probably apply a fresh coat about once a month. But you might also consider using a dry-season lubricant and a wet-season lubricant if you experience big seasonal swings in weather. 

Ultimately, though, I think it comes down to how much you ride and what kind of relationship you want to have with your bike. Some people identify strongly as a cyclist and get a lot of personal satisfaction out of optimizing their ride and keeping their drivetrain and bike perfectly clean. You can certainly go the other way, I think the majority of people who own a bike will probably never clean the chain. Most people probably never wear through a chain, either. So if you do either, I think you're on the right track.