Half Through What?

[Quercus]

Gandalf moved his chair to the bedside, and took a good look at Frodo. The colour had come back to his face, and his eyes were clear, and fully awake and aware. He was smiling, and there seemed to be little wrong with him. But to the wizard’s eye, there was a faint change, just a hint as it were of transparency, about him, and especially about the left hand that lay outside upon the coverlet.
‘Still that must be expected,’ said Gandalf to himself. ‘He is not half through yet, and to what he will come in the end not even Elrond can foretell. Not to evil, I think. He may become like a glass filled with clear light for eyes to see that can.’

I’ve never quite understood this part of the book. What does Gandalf mean by ‘he is not half through yet’? At this point, Frodo had not yet volunteered to take the Ring into Mordor, so how does Gandalf know that Frodo’s not half through his quest ? Or is Gandalf referring to something else that Frodo is not half through?

And what about the reference to the ‘glass filled with clear light for eyes to see that can’? Is this why a number of people notice a glow or elvish quality about Frodo later on in the story? Or, again, do you think that Gnadalf was referring to something else?