Harry Enten at 538 does a nice job pulling together the data from the summer front runners in Iowa going back to 1980, New Hampshire and national polls. Let's see how Trump stacks up:
RCP average of 19.3% (down from the peak of 22%), he is in Dick Gephardt zone. Which President was he again? You really have to go to Bob Dole's 34% in 1996 for it to mean anything.
Now let's look at New Hampshire:
RCP average of 24.5% (down from a peak of 25.3%) which I think is being skewed by one Gravis Marketing (who?) poll which gave him 32%. Regardless, he is not even in Howard Dean territory and honestly seems to be doing the Dean Scream on a weekly basis.
Now let's look at the nationals:
RCP average of 22.5% (down from 24.3% at the peak) Trump at least finds himself at a place where historically someone was able to win. Though obviously I don't remember a President Hart or Cuomo so take it all with a grain of salt.
This of course is not to say he has zero chance because he is leading now. I'm sure if you that if you run the numbers, leading in the summer before the primaries gives you a better chance than being a candidate that is not leading. It's just not even close to a done deal. Thank goodness.