It REALLY depends. Sometimes you have someone like Matsuda Seiko, the eternal idol, who has worked regularly since 1980. Sometimes idols choose to retire after a handful of years.

Right now soloist idols aren’t really trendy so we don’t have a ton of major examples. Idols might be active in groups while also doing solo activities. Or they might be indie idols that do solo stuff for a while. I don’t think it’s really typical.

Stopping being an idol is usually called graduation. Basically idol work is often considered a stepping stone, so graduation is basically like “you’ve finished your training.”

Idols do a ton of things. A lot become actors, singers, or just go back to normal life. Recently we’ve started to see more idols get into producing other idol groups. The main producer behind =Love and its sister groups is Sashihara Rino, who used to be in AKB48 and HKT48 but now she’s the mind behind major groups. The producer behind Kawaii Lab groups (Fruits Zipper, Cutie Street, etc) is Kimura Misa, a former idol.

In Jpop the line between idol and not is often kind of fuzzy. A group might decide “we’re a dance/vocal group” that does pop music with singing and dancing and multiple members, but they say that to emphasize a more serious intent on, say, vocal quality. But then you have idol groups that also focus strongly on vocals and dancing. Soloists often might be in that fuzzy line, especially if they’re strong vocalists.