I wouldn't say always on my mind, but I think about her a lot through out the day. Most things I see remind me of her, or make me wonder what she would think of it, or if she would like it. I'll see stuff about issues she finds important and it makes me think of her and what she would think about it. I used to be one of those guys who would develop a HUGE crush on a girl I'd never even talked to ( the kind of nice guy you see in comedies ). I would think about her everyday.
Recently I fell in love with a girl like I never thought I could. Unfortunately, due to various personal circumstances we weren't able to see each other or talk to one another for about 3 months. I felt like I longed for her EVEN MORE and it seemed as if everything I saw or did reminded me of her. When I saw other couples it made me wish I had her by my side. I would notice her dream car, or her favorite flower, stickers for bands she liked; reminders of her were EVERYWHERE.
Granted, there are men out there who don't seem to care at all, but I'd say they are the exception rather than the rule. Most men may not feel AS strongly as I do, but we do think about you a lot more than you probably realize.