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 :)
Answer: Be organized and understand what you want to do and what you can do
Explanation:
Schedule your time in a way where you are satisfied and you can achieve with ease, don't make your schedule what you want to achieve and be successful with, schedule with things that you can do, don't do more than you can handle.