Just wondering, has anyone ever done an experiment on how long it takes on average for horses of different ratings and/or breeds to level-up? Like, we're talking a "real" research experiment: start with X number of horses of each breed and rating that have had zero training, distribute them evenly between riders to minimize standard error, etc.?

Yes, I did several experiments like this in the past 10+ RL years.

At the end of a lifetime, all horses of any rating are able to earn almost the same amount of money.

The ones with the lower ratings take a lot longer to level up - but once they are in the winnings they will stay there for much longer. A horse with an E or W rating will sometimes level up even before they are in the top winnings - and they are SM at about 13yo with SD training.

The horse with the sub rating will not even become SM eventually before it dies. But if it gets shown on a regular bias whenever it is ready, it will be able to earn almost as much as the higher leveled ones!

Training times will vary, depending if they are trained SD or AD.

While a WWW AD horse will level up within 4-5 weeks, a sub/sub/sub AD horse will probably level up within 12-14 weeks.

All the others are somewhere in between and this is why I consider good SSS or even lower PPP to be the best rating for showing.

That's interesting to know! I'd be interested in doing some experiments of my own as a side project, but don't know if I want to sink the ebs into it just yet since I'd want to purchase a couple separate barns and need to fill them with horses to do so. If I sold my crop fields that I, unfortunately, don't use, that would get me a good start though. :P