That's been my experience with many sansevieria as well. If the main cluster stops growing, it usually means it's just developing a pup.

I didn't realize that Moonshine's had so many leaves per cluster and that's all mine did all summer, produced more leaves within the existing cluster. I brought all my plants in before the hurricane that turned out to be more rain than anything else for us and realized that it finally had a pup coming up. The same holds true for the braided cylindrica that I unbraided and repotted, it finally rooted well enough to pup. I think its fun, makes me feel like I'm doing something right, even though the anticipation can kill you at times :)

I find that my Sans start pupping in winter, and the pups do their growing in spring and summer. I'm looking forward to seeing how my pots fill out this winter

Hmm. This has got me wondering about my trifasciata, which I've been rehabbing for about a year after a long period of neglect. As the clusters have slowly grown new leaves, I've been pruning off the weak, pale ones, but that means the base of the plant is pretty ugly and I would love some pups to come in and hide the old leaf scars. Is my pruning getting in the way of that? Will it just keep stimulating growth from the cluster instead of pup growth?

I find Sansevieria trifasciata ‘Laurentii’ and the non variegated ones reasonably vigorous if conditions are favorable( Think sandy loam and bright light!). My slowest cultivar is my Golden Hahnii. It pups reasonably well when happy, but sulks under less than the most favorable conditions.

Elena, Sansevieria love heat. Keep them warm and they will grow faster.

That seems to be true. I have a large south-facing picture window with a forced air heat vent right below it. I moved my sans closest to the vent because I figured they wouldn't mind the warm, dry air, and three out of four are making pups!

The problem with close to the vent is heated air is very very dry, but it seams to be working for you so just watch out for signs of over drying. Many of my plants are above a radiator simply because many of my windows have a radiator under them- nothing I can do about it. Some plants will not tolerate those positions and have to be moved away from the radiator, others need to be watered at least twice per week, some don't mind it much al all (mostly cactus).

I do have to water a little more often, but humidity runs high here even with the heat on, so it's not too hard on the plants.