When dust comes out, it's time to stop

Great question.

Essentially if its not interfering with your life in a meaningful way, you're fine.

If it causes you to miss work or if you're unable to wait until you get home and end up doing it somewhere its not acceptable, then its an issue.

Or not being able to stop or reduce the frequency can also mean its an issue.

However, rather than listen to some nobody in the internet, you'd want to consult a medical professional.

21 times a month should be good enough.

More shouldn't be a problem either, since once every day reduces the chance of prostate cancer by 36%

Are you numb, bleeding, sore or neglecting other activities like friends, family or your job?

You're probably fine. Whole area's kinda built to do that pretty regularly. So same rules as exercise.

It does vary from person to person. Whats reasonable for you may not be reasonable for others. Libido as well as age is gonna be a factor. Some people do it multiple times a day and others are a couple times a week.

Again as long as its not hurting you or others, go for it man. Like others in this thread mentioned, in moderation, its a healthy thing to do.

That's the beauty of public transport

Boss makes a dollar, I make a dime, that's why I crank my hog on company time

Still doesn't look like anything definitive. Words like may and could are littered throughout the article. It looks like the main evidence was a questionnaire.