Milo FEELS like he has been gone several weeks. Remember that this is a fantasy, and time can be manipulated. The last chapter tells us that Milo is suddenly concerned that people may be worried about him.
"...it suddenly occurred to Milo that he must have been gone for several weeks." (pg 253)
However, when he returns home, he notices that it is only six o'clock, and he has only been gone an hour. No one has even noticed that he was gone! He has done so much and experienced so much in that hour!
" 'And it's still today! I've been gone only an hour!' he cried in amazement, for he'd certainly never realized how much he could do in so short a time." (pg 254)
Note that he also doesn't tell anyone about his adventure. It would be hard to explain a spelling bee, a dog named Tock, and the Humbug, not to mention Rhyme and Reason, King Azaz, and the Mathemagician. The Phantom Tollbooth has done its job, and it disappears, but Milo is a changed person. He has gone from a bored child with nothing to do to a boy who would like to take another trip but,
"...I really don't know when I'll have the time. There's just so much to do right now." (pg 256)
It isn't wrong that he kept this a secret. It would be much harder to explain it than to keep it a secret. Who would believe him?