The first time he thinks about it is two days before Kurt is supposed to leave for New York, when he's helping him pack all his stuff and carry the boxes down the stairs. Well, it's not the first time he thinks about it, but it's the first time he really considers it.
He's very careful not to tell anyone what he's thinking though. He can already hear the reactions he'll surely get from family and friends.
"Are you out of your mind?!"
"But you're only 17 years old!"
" How can you possibly know this is what you want for the rest of your life?"

The thing is, he is sure this is what he wants. He can't rationally explain why, but he's 100% certain that he and Kurt are meant to be. If only he could be just as convinced that it's what Kurt wants too. Of course he doesn't doubt that Kurt loves him. It's just that sometimes, Blaine gets scared. Scared that when Kurt arrives in New York, he'll find someone better. Someone more worthy of his love.

They've talked about Blaine's fears and about doing the whole long distance thing before, more than once, and every single time Kurt assures him that there is no one in the whole wide world that could possibly be more deserving of his love than Blaine. Unfortunately, fear is often irrational and refuses to be put aside so easily.
So when the thought first crosses Blaine's mind, he's absolutely certain that he wants to do this. He wants to propose to Kurt. However, he's not so certain what Kurt's reaction will be. So he waits. Someday. Someday he will. Today is just not that day.