This is a very tricky question as you can get a lot of answers that will be partially correct.
The best answer in my oppinion is between the range of 7 minutes to around 15 minutes.
The reason why saying just one value isn'the best idea is because, if we have two groups of people, the average for the first group might be 7 minutes, and the average for the second group might be 11 minutes. So it's hard to say the average is 7 minutes. It's better presented in an interval where we can find this number :)
I couldn't find that much but I found the same question as yours and this is another person comment. :3 hopefully that help :D
A diet that can lower your stress looks like something with a lot of fruits and vegis. Making sure you get a least one hour of exercise a day. Plus drinking tons of water and a some juices. Foods I'd consider would be pineapples, strawberries, bananas, cucumbers, carrots, and etc.
If an example is what you need i'll try my best to help. Say for example, you're sitting in class and you're doing a group project and you want your best friend to do it with you but (s)he thought you had meant that you wanted to do it with someone else, because of poor body or facial language.
Does this help at all?...
True, you can improve your health and help others by reducing secondhand smoke to them