Naismith's Rule, allows an easy calculation of the time taken for a hill walk, for a reasonably fit and steady walker: 15 minutes for every kilometre of horizontal distance, plus 10 minutes for every 100 metres of ascent. The time should be calculated for the slowest person in a walking group.
Corrections for descents
going gently downhill - 10mins / 300m of descent
very steeply downhill + 10mins / 300m of descent