It's quite a lot, but I wouldn't put it in the realms of addiction. It seems to be quite typical compared to even the personal (i.e. non famous) people I'm following; the higher profile accounts I follow tweet multiples of tens per day.

Not for someone who's fulltime job is writing and reading fiction.

Writing 3 lines of text a day is a lot for a professional writer?

It's really not

King writes 300-500 page novels at an age when most people are retired. 240 characters here and there is not evidence of much effort for him.

I mean we could ask Stephen King I guess, he knows something about the topic.