About 25 to 50 percent, depending on which Polling Aggregate source you’re using for Biden currently. Which would presumably improve with another candidate.
Meanwhile, RFK Jr., the highest polling of the third party candidates, has less than 1% chance of winning enough electoral votes.
However, my original point wasn’t that a Biden replacement would do better than RFK or a third party in the general (though they certainly would), but that if you dislike Biden, him being replaced is more likely than a third party candidate ever winning.