Firstly - this question needs context. And that is determining what is meant by 'winner'. Someone can play a hand and win. Or several
hands (session) and win. That is rather useless info. In my opinion,
a poker winner is someone whose long-term bankroll is in the black. Ups and downs over the month and years are part of the journey.
Obviously, only the
online poker rooms know the answer to that - through the data they have to have on each client. Poker tracking software would also provide some insight, but not much. The biggest challenge is THE RAKE. In the long run, all players are paying the rake which eats into profits.
I won't estimate what the percentages are because that is just guessing, in which case my estimate/opinion is worthless.
There is only one place where I can reasonably estimate - my weekly live game among a group of 6-10 friends. Only 2 of us are long-term winners. We both keep our own records. None of the other players keep track. They play for fun, and it is fun taking their money. In this context I am in the 5, 10, 20% of players who are winners.