PE requires time, dedication, desire, privacy, and a healthy life style for it to be effective.
How much of the population fits that description? And how much of that small percent has stumbled upon PE and decided to take it seriously?
In terms of world population I am betting that it is under one one-thousandth of a percent considering that two thirds of the world has a hard time staying alive, much less has the time to pull on their wiener for an hour a day.
If we want to only talk about tier one countries (USA, Canada, the EU, a few countries in South America) in which people would have the inclination and time for such a thing I am willing to bet that it is still below .5 %.
Now, if you want to narrow it down even more to the people who are sexually active, inclined to worry about something like dick size, and have the time and dedication to make it work ( ie. the people you are competeing with to pick up that hot chick at the bar ) then the number gets higher.
There are plenty of pay sites out there. If you really wanted to find out you could probably find the total number of subscriptions to those sites, the number of subscriptions to sites like TP, and so on, and get a reasonable approximation of how many people know / care.
There are probably internet polls too, but those are naturally biased because anyone who stumbled upon an internet PE poll is probably already inclined to be inquisitive into the subject.
Cheers,
Whiskey