"I am not long for this world."

This is Alice: On the ironic level, where language is as dull as imperception, the expression is simply a cliché.  This is how the boy hears it, for he says, "I thought his words idle."

But at a higher level of perception (moving up the levels), it could be prophecy, and, therefore, mythic.

Cabe has a little explanation of these levels if you want a quick one 

We can analyze the sentence to ask much more of it, if you wish.