It varies, based on how much we like them, who they are, what kind of relationship there is, how long it's been going on for, and who we are. I think I can say across the board that it's mostly fiction that no one thinks CONSTANTLY about someone they like. There's other things in life to worry about, being that obsessed is probably a bit unhealthy. When I was genuinely in love, and I mean as an adult, not a childish infatuation, it was more like... Anything that could possibly remind me of her made me think of her. I didn't need to, I wasn't obsessing, I would just get reminded of her by anything and everything, because she was close enough to the surface of my conscious thoughts. Like, I was getting oral surgery at one point, and because we were both a bit medically obsessed, I thought she would think it was cool and took a picture of the surgery to show her. Because really, even the most random things and at the most unlikely times.
Ahahahahaha, I love the fact you asked this question. I should ask the same thing but for girls. Personally, I pretty much agree the above two answers to a degree. It depends on the kind of guy you think is interested, but for me they envelop quite a bit of my mind. Sure I have life and other things to attend to (Work, hobbies, friends) but they're in the background, and I'm hoping they're thinking in the exact same way. It's nice to know that you're in someone's thoughts, and on my end I can guarantee you.. yes you are.