When it’s late afternoon and you’re wondering if you should make camp or not, use this trick for estimating the time until sundown.

Extend your arm and your fingers toward the sun, then bend your wrist. Each finger represents about 15 minutes. Leapfrog your other hand if needed to estimate times more than 1 hour.

This trick works if the horizon is flat or a mountain ridge line, it doesn't matter. Note this is an approximation, and varies a bit depending on your latitude.

In my neighborhood in the Pacific Northwest, a good rule of thumb to total darkness is add about 30 minutes to the sunset time. So, if your hand (or your smartphone) tells you sunset will be in one hour, you have about one hour 30 minutes of usable light. This will depend on your latitude, test it in your local area to be sure.

The same applies to usable light in the morning; subtract 30 minutes from sunrise time to find the first usable light. For alpine starts, this can help you plan your wake up time, as being on more technical terrain when you can see where you're going is usually a good idea. 

Example: from your camp, you have an hour of easy walking on a trail before some technical climbing begins. You and your partners want to have a fairly leisurely hour to get ready after you wake up. Your phone's weather app tells you that sunrise is at 6:15am. 

Subtract 30 minutes from 6:15, and you get first usable light at 5:45. Subtract two hours from that, and you get an approximate wake up time of about 3:45 am.