It depends. I haven't seen my fiancee for about 2 months, so... I'd say about 4 times a day. Hell, I damn near cannot sleep without "mixing up the man batter".
However, when she is about, I always feel it's somewhat rude to "fiddle the flute" when there is a perfectly willing and lovely partner to partake in, so I don't.
What time of the day? Well, like I said, I can't sleep if I don't "finger the pennywhistle", so there is once. And then more or less any time I am working at home or interneting and I see something attractive on ye olde net, or I look through my old photos and see pictures of my fiancee, I just get the urge to "evac the sack".
I have "painted tissues" during my teenage years more or less at the drop of a hat; I remember one of my exes giving me a "hand shandy" at school once. Embarrassing to think of now.
I tend to think about sex. If my thoughts ever go off topic, I tend to have to start from the beginning.