I thought I already posted a reply to this, but since it isn't here... *shrugs* getting senile.....

It takes practice. In high school, during the summer, I either worked on farms or for my dad when he owned a house painting business. In either of those, wearing a watch obviously is a bad idea. So, I got to a point where I could say what time it was without digging out my watch.

Keep in mind, that is +/- 15 minute accuracy. That's a 30 minute window.

Realistically, here is what you NEED to know: if the sun is less than three hand widths above the horizon, you need to be thinking about making camp, becuase you don't have a lot of light left. Other than that, eat when you are hungry, drink when you are thirsty, take a tenner and change socks at highest sun, and let the sun wake you up in the morning if the cold didn't kill you first.