Is there a relatively reliable way of calculating the time until sunset, without any specialist equipment, when I'm in the wilderness?
One method I've found to be particularly reliable is the finger method - hold your arm straight out in front of you, facing towards the setting sun like so:
Place your hand so your little finger is level with the horizon, and your fingers are stacked on top of each other. Each finger represents around 15 minutes of sunlight before the sun sets behind the horizon.
This method can be used relatively accurately up to two hours before sunrise, by using both hands.
See also this page.