If you're talking about 2012, I could see him receiving a maximum of 40% (and that is probably way optimistic). If you're talking about 2016 or another election in the future, I could theoretically see him winning 50-55%, but probably no more than that.
I agree that it won't be higher. I actually think the ceiling is about 45%. There won't be an overwhelmingly monumental appeal.
The only thing that could allow that to even happen is that Hispanics just come to the center of the American political spectrum a bit giving the Republicans some votes.
I think that despite Rubio's hardcore conservative positions, many typically Democratic Latino voters could vote for him due to identity politics. I'm sure a lot of Latino want to see one of their own elected President, and thus I think many of them who typically don't vote for the GOP would vote for Rubio (just like how some Republican blacks voted for Obama). Thus, in a neutral/good year for the GOP, I could see Rubio winning a majority of the Latino vote. Besides, he could always de-emphasize some of his more controversial positions, if necessary.