It really depends on how stressful I am.
Sometimes just chilling out with a good book or nice music helps, and sometimes even cycling to the mountain and back will do no good.
I usually do not have these intensely stressful situations, but in case I do, and I want to do an experiment, which I did in the past, to see which distance can help me relieve my stress, I would say, It is around 20 kilometres. Both ways, I mean, ten one way and by the time I am on my way back, the adrenaline, the nature around me (because I prefer cycling in nature), plus all the thoughts that set themselves in a perfect order, help a great deal.
I guess the think is that one has to be outside, feel the wind on their skin, or the Sun on their back, to get rid of intense feelings and be stress free. But, it is just what works for me. Everyone is different.