It's been several years since our kitties passed, but when we had them, we bought the largest littermaid box available at the time, which I believe was the mega. We had to get one because when our cats got older, for some reason, one of them would absolutely not go in a dirty litter box, even if he was the only one to use it. We had to find a way to get him to use the box or he was going to have to go, and after 14 years, we weren't really wanting to do that. Having the automatic scooping box helped tremendously with that cat and we loved it. My oldest son (now 23) had primary litter duty and he just asked me a couple days ago if we still have the littermaid because he's tired of scooping after the cats he has where he lives now.
Essentially, we would empty the waste receptacle once a day and maybe had to clear a "jam" less than once a week. The jams were usually caused when the clump was too big to easily fall into the receptacle, or the receptacle was too full and the waste wouldn't dump properly. That happened when we would forget to dump the receptacle once a day. :-) We could tell when it was jammed 'cause our picky kitty would start looking around for someplace to relieve himself. We'd jump up to go see the litter box and sure enough, it would need to be reset.
Our cats were both over 15 pounds but they had no problems with using the box. If we got another kitty, I'd definitely use one again.
Blessings,
N.