I know a lot of you put a lot of stock in the timeline - I don't. The reason that I don't is in the books themselves there isn't a definitive timeline of the time that they spent in Narnia - now there is for their time away from Narnia (in PC, they state that it had been about a year since their return from Narnia - VDT states that it had been about a year AND Caspian says it had been three years Narnian time). It isn't even really clear when the Pevensies were sent to Prof. Kirke's. All the book says is during the war, because of bombs. I get the very distinct impression in PC that WWII is over. You don't have to agree with me - but that's just what I see when I read them.

Now, as to the question of why they didn't go back immediately. Well, when they first got there - they wanted to explore a little bit. Then Edmund betrayed them and they were intent on rescuing him. When Edmund came to his senses and returned to them, Peter and Edmund were focused on defeating the White Witch. Lucy and Susan were following Aslan and then helping Him gather the rest of the troops (oh, He brought those who had been turned to stone back to life - but Lucy & Susan helped Him find them.)

After Aslan killed Jadis, the four Pevensies were coronated as Kings and Queens. We don't know exactly how long it had been since they all arrived - but the way it reads, it was days, not weeks. Very eventful days! So, they really hadn't had time to think about going home. Now, put yourself in their place - an evil Witch was just killed by a Lion (who had been killed by the same witch - and He conquered death) and He makes you and your siblings Kings and Queens of the land He just saved. Are you really going to say, "Thanks so much, and I'm really very flattered that you picked us to rule the place, but we need to go home. Oh, we know that some of the witch's army escaped, but the Narnians will have to fend for themselves - we're out of here."

Aslan made them rulers - that isn't exactly something that you take lightly. I don't think I'd tell Aslan - "see ya! I'm going home now."

I also think that they might have thought about their parents and home in the beginning, but after a while, Narnia was their reality and their responsibility and Earth became like a dream.

Plus, there is every indication that the doorway through the wardrobe wasn't always there - otherwise, someone who belonged to Narnia might have stumbled on it at some point during their reign. So, even if they had tried to go back at some earlier point, they wouldn't have been able to until Aslan was ready to send them back. (And by the time He was ready, they had been there so long, they really didn't think about Earth much - until they passed the lamppost.)