I would say, forums take a lot of time if you are to be a regular. That may be possible at a time of one's life and not later.
I spend a lot of time reading several forums, but as soon as I stop actually posting, I see a huge reduction of time past on them. For me at least, posting is what is time consuming, and psychologically tiring.
Ultimately, the forums I really spent a lot of time posting on where forums I was really involved in (either as mod or as someone writing regular articles etc) i.e. if I was really considering it as a job. The longest I've been regularily on the same forum is 18 years and counting. A few between 5 and 8 years (most now defunct).
But for most forums, after a while I move on, at least being an active member. Generally I would still read and sometimes post, but just not that frequently.
For some people, there is also the fact that their centers of interest may change. That does not happen to me, but I ADD new center of interest as I grow older, so I have to redistribute my time between more forums/communities.