From my own personal experience, when doing a strenuous bike ride, I have made it a habit to spend at least 10 minutes post activity to spend cooling down. I think that 5 minutes cool down per 1 hour activity is a good baseline. When I am biking, I will go into a much easier gear and do high rep’s for that amount of time. A runner may find it beneficial to walk for 5 minutes or so after an hour of running. This is beneficial in helping the body to rid itself of any metabolic waste from the muscles, therefore helping to reduce or eliminate DOMS (or delay onset muscle soreness). I would prefer doing this over stretching after an activity. This is especially important when starting or working towards a goal with an activity. So, if you are experiencing discomfort or muscle pain post activity, give this a try. Remember to stay hydrated as well up to 6 hours post activity.