My game has only been public for about three months, so the numbers are a little low...
Out of 855 created accounts, 200 have not verified their email address. 32 of those accounts never logged into the game after creating their accounts (so the verification email never got sent), and a few came from the same IP address. Overall, 163 distinct IP addresses created accounts and received verification emails but never verified their account.
I've found that having the text "note: You will be asked to verify your email address" on the registration form significantly reduced the number of bad email addresses.
Also, the wording of your verification page is important. Before I added a verification link to the email, and verification was done by hand, players typed completely random shit into the verification form (I log bad verification attempts to help problem players through the process). I eventually added a paragraph long description of what exactly to type into the verification field and now only one third of the time do I get completely random shit. Now that I have a verification link inside the email, I almost never have a problem with email verification.
1/4 of created accounts sounds like a lot to lose, but I currently have 0 obvious cases of multiple accounts, and about 5 questionable pairs of "brothers" who I watch carefully, which I consider much better than the game my system was originally designed for, which had a number of players with 20+ accounts.