Nope not really, just a random occurence i think. Unless of course I'm going through a period of particular hornyness, and theres a particularly horny scene on TV that can start it off. Never comes at a certain time of day tho.
Well... habbits are habbits. No matter how sexual they might be.
If you make it into a habbit to masturbate by a certain time each day, then it's hard to stop after a while.
It's like waking before the clock rings when you work. Your body knows, soon it's time.
i'm sure it's the same as for men. Have to be alone or during sex for sure. And if I am and feeling particularly horny, from a movie or a dream, or just a thought.
