my older generation Riedell smooth leather boots [including the Royals, which at the time were the top-of-the-line Riedells] formed creases within several sessions (and, from discussions with skate techs, coaches, and other skaters, this was fairly typical behavior)
I'm very puzzled by that statement. Unless there was something special about those boots, or they fit very badly, or weren't tied tight, that shouldn't have happened. I'm not basing this statement just on my own experience - I've also had a lot of discussions about boots with several very top-end skate techs. I've also watched what happens to rental boots at the rink where I've worked.
What breaking in should do is allow the layers of leather fiber to slide against each other at the points of the bend, by tearing the substances that connect them. But that doesn't mean you have to develop a visible crease.
Even relatively low level rental boots won't develop a perceptible crease for most people within a few sessions, unless the boots don't correctly fit the foot, or they are worn too loose. That's because a concave break-down crease can only develop if there is an empty space inside the boot for the leather to bend in a concave manner. Of course given time - a fair number of years, and hundreds to thousands of hours - a crease can still form - when leather bends, even if the leather is well supported by pressure from the foot on the inside of the boot, the outside of the boot will bend concave while the inside of the boot bends convexly. The leather can break in - become soft at the points of the bend - much more quickly.
OTOH, if the boots don't fit, e.g., they are too large, are of a shape much different from the foot, or are not tied tight, boots will develop a crease very quickly. Under those conditions 1 session can actually break down many rental boots. It's easy to see why - if there is empty space next to the foot just below the ankle, the leather will be forced to bend concavely (inwards) there.
I have creased and broken down one specific type of boot that quickly. Out of curiosity, I bought well-used boots of very thin (1/20 inch?), supple (soft as a glove) leather. At one point, fairly long ago, they used to make many boots, almost knee-high, that way. The boots were well used, but no crease had developed. I tied them very tight, which forced them to fit well against my feet, but they still broke down in one session.
My first boots were somewhat used smooth leather Riedell 220s (fairly low level boots), of a fairly old generation, which at the that time were single-layer leather, about 1/8" thick. They didn't fit all that tightly - I didn't realize they should, though they were supple enough to conform to my feet, and happened to otherwise match fairly well the shape of my feet. I regularly oiled the leather on the inside, which lubricates the layers of leather that need to slide against each other when the leather bends (not practical to do on many high level boots). They broke down in maybe 1.5 years - maybe 250 - 500 hours on the ice. They creased within about 1/3 of that. Certainly not within a few sessions. At that point, Don Giese, a very well respected skate technician, reinforced the boots, and effectively made them smaller, by gluing a carefully shaped layer of leather inside the boots, something he had done for other people's figure, hockey and speed boots. That worked for maybe 3 - 6 months (?), but the boots were really already broken down.
My next boots, custom (but not well fit originally, by a skate tech I would never use again) very well made Klingbeil
"soft" smooth leather Dance boots, didn't start to break down and develop a crease for about 9 or 10 years. At 10-20 hours/week, they broke down in very approximately 4500 - 10000 hours on the ice. That's NOT typical - I worked very hard to improve the fit, and I barely jumped at all, and Klingbeils were well known for durability. They didn't even break in for more than the first 6 years of that - and only after I deliberately forced them to break in by a combination of chemical, thermal and physical means. The reinforcing foam between the inside and outside leather layers had crushed down - but Klingbeil rebuilt them for me at the 6 year point (2500 - 5000 years on the ice), by replacing the foam. But the leather was still too stiff for me. (I was taught that good ice dance boots should bend perceptively sideways, to the limit of your range of motion, because you use sideways bends to create deeper edges. In contrast, freestyle boots should hardly bend sideways at all, to better protect your ankles.) I continued to use them for almost two years, though they no longer supported my ankles much, and I probably shouldn't have.
Mike Cunningham, one of the world's best skate technicians, told me that even the world's highest level freestyle skaters, doing triples and quads, typically take about a year or so to break down their skates, if they are fit correctly, though there is a fairly high degree of variation in that.
I don't think they should start to create a visible breakdown crease in much less than that - maybe 1/3 to 2/3 of the way through the boot's practical lifetime.