Constantly. I have no clue how this perception of 'guys don't fall in love like women' came into place to begin with. It's all dependant on the person really.

For me personally, when I'm in love, its almost like she's become a source of light for me. Everything about her is...magical...in a sense. To figuratively put it, it's almost like she has a halo-ish glow around her, where you can't even consider anyone else. The thought of her smile you remember from that one amazing day will light up every day afterwards. How she talks, how she reacts to you, it all has an adorableness and whimsical nature to it that makes you smile almost instinctively thinking of her much later while working. You see her in your bed sleeping and you think about it a day later, a week later, a month later, how it made you feel like the luckiest s.o.b alive. You picture walking in the rain, her palm in yours, her doe eyes looking at you upwards. You picture walking on the river bank in the winter, hoodied up, she holding onto your arm for warmth, her head on your shoulder. You are just waiting, to experience it all with this amazing person you can't believe you found.

So yes, guys do fantasise about their SO a lot when they're in love.