Most Helpful Guy
When I was younger, up to about 18, I would fantasize about actual women/girls I knew. I'm not exactly sure why, but that actually changed to a more generic fantasy (of what I wanted to do, see, and have done to me, etc) where the girl was just..."the girl in the fantasy."
I found it disrespectful to fantasize about people I knew, and pointless to fantasize about people I didn't know, because I wouldn't know how they'd act.
I do still fantasize about fictional characters (books, movies, etc) though. Not real, so I don't feel like I'm disrespecting an actual person, and you can incorporate how they'd act based on what you know about them as characters.
As for how often? Look, even if I go a month without touching myself, I'm still thinking about sex every 5 minutes.