I'm a rotational player and I have a lot of households. This also means that a lot of my own sims walk around in the community worlds which makes them more interesting to me. If a sim marries into a household that I'm playing, they "join" my rotation. If I get uninterested, I'll make a household unplayable and then just leave them as townies wandering around. I don't play with mods. I switch households whenever I want to and I play with aging off for playable households. I tend to freeze time as I'm moving around between the different households so that it's like everything is happening at the same time but I'm seeing all of my sims evolve. Every so often, I'll age up the unplayables but lately I haven't because I might want to make a few households playable again later to see if they interest me more.
To answer your questions:
1. I started this rotation by deciding not to pick an heir and to follow two brothers with different lives. I then decided that the town needed a lot more characters. I made potential spouses and then decided to follow some of the ones who weren't picked. In the beginning, some of my households joined because I wanted to see what would happen to some of the rejected love interests, would they ever find love? Now, I just have a whole lot of sims and I'm generationally farther along, so I now switch between various characters. I choose to switch when I get to a stopping point with a particular household. I try not to let one household age up way more than the others before going back to play them. I make myself check in on specific households every so often. I'm just able to keep track. If they were to get really boring I would quit playing them but usually, they get more interesting when I do. No one is an elder when someone of the same generation is a teen, if that makes sense. It means I get to experience a lot of different households with different homes, lifestyles, etc.
2. They are all different types of households because it's a legacy rotation, so they might be single, then get married, most of them have at least one kid. Few of them stay single long term but they might go through a long time that way, in which case, I'll usually turn aging off. If I like a household or am working on some part of their lives, I'll turn aging off. Unless they have kids, I turn aging off in university. That's one rule of mine, when they have kids, the aging clock goes on for them. That's when time definitely moves on.
3. If you are wondering how long I'll play a household before I switch, it depends on what is going on the game. I have Seasons with a long span on so I'll pay attention to the shifting weather and holidays. I might just check in with a household for a few days or I might spend weeks with one. With university, I obviously need to play at least a week at a time. If you are asking about how long it keeps me interested, I've had my rotation for years, but I have a life, interests and responsibilities. It keeps me interested when I play because the characters are "mine" and I like seeing what happens to them. It's like my own tv show in which I don't need to worry whether it entertains anyone else. I think I spend a lot of time on character development for sims I like.
4. Newcrest is a centralized neighborhood that is mostly non-residential. I only have two households there and instead it's a neutral place where sims can go to hang out. I have a cafe, parks, a spa, the lost garden of time, a cinema/bowling alley, a different kind of library, bars, clubs, several restaurants, etc. Not sure what to do? Go to Newcrest. I don't have a specifically favorite world. I like switching between them and seeing how the neighborhoods develop.
5. How long it takes especially with as many households as I have. I've sometimes wondered where it is going but it's entertaining. When I get into it, I'm generally happy. There are times that I've made myself play a household that I was bored of. With a big rotation, I need to just say, "It's ok to just make them unplayable," and to just concentrate more on the households I like. I don't need to explain why I like them, I just do.
6. Eh, I like switching between different households and phases of life. I'm mostly interested in my characters and I like having very different visual environments. I just don't want anything to be too repetitive.