One - there is the travelling. It seemed like our character/companions were walking on foot, not by horseback or carriage.
Two - Didn't Morrigan say that our character was out of it for 3 days because they were badly wounded and it took Flemeth quite a bit to heal them? (I could be wrong but I could swear that they did mention our warden being out for a good while).
Three - Redcliffe didn't happen in one day. You arrived in Redcliffe (and depending on what you choose), managed to get into the castle the next day. So it took two days considering the fact you needed to fight your way through the battle. Then of course there was finding the Ashes. Which meant you had to go to Denerim then head up the Frostback Mountains and then fight your way through Haven and through the Sacred Ashes. That - logically - would have taken a few weeks at least considering the amount of travelling you did.
The Circle was in the middle of a freaking lake so it probably would have taken a few hours to row over to it (remember - they were trying to keep the mages from escaping, exhausting them as they swam for the shore would have been a plan). Not to mention you had to fight your way through the Circle - which probably would have taken a while too. Then there is the Fade where it was mentioned that you were stuck in the fade for a good amount time. (Not enough for your body to be weakened considerably but enough for time to pass).
Then there is the Forest - you had to go through the forest before you found the Dalish. Then you had to go through the forest before you made your way to the ruins. Then you had to fight your way through the ruins. Then (depending on the choices you made), either went off to find the Keeper and bring him back, or fought Witherfang or led the werewolves through the forest to the camp and slaughtered everyone. I am guessing that took a while as well.
Not to mention Orazammar (which is considered to be one the longest sidequest). Depending on the choices - you could have made your way into the Deep Roads to pass on a message of the 'falsified' documents. Then you had to Shut down the Carta by killing Janka. Then you had to look for Paragon Branka which meant you were in the Deep Roads.
You could have possibly been in the Deep Roads for weeks, maybe a month or so considering how long and maze-winding the place was. Not to mentioned you would have been slowed down by the constant fighting.
Then you would have to make that travel BACK to Orzammar and crown a King and wait for them to give you the resources you needed. (You would have also needed to recover).
Then you needed to make your way to Redcliffe to tell creepy-Eamon your 'victories' which then you headed to Denerim, where you are immediately sucked into having to search the Queen, fight your way through the Estate and kill He-Shall-Not-Be Named (and Vaugh, if you were delighted by the opportunity), then of course you had to fight/sneak your way out of Fort Drakon.
Which probably took a while considering it took Anora that long to get to Eamon (especially since you showed up pratically right behind her).
Then you had the Alienage to contend with, and getting support/allies in the Landsmeet against Loghain.
Depending on your choices - Loghain either needed to undergo the rite or everyone made their way straight back to Redcliffe and thus you either had the ritual to contend with or not.
Then you find out that Ridoran was actually wrong about where the Archdemon was heading and thus you had to make a long trek back to Denerim (Seriously dude - you though the Archdemon wouldn't lie?) then you had the battle.
Of course - this doesn't include the sidequests and DLC contents.
So, logically, it could have taken a year to gain all the allies you needed against the Archdemon.
For the Archdemon itself - I just thought it needed time before it could surface. Maybe it needed to gain strength/awareness before it could venture outward. It had been a sleep for a good long time so it would make sense it wasn't straight up ready for battle.
Calian beliving Duncan probably was the reason how they managed to get a head start on the battle. With Alistair and the Warden armed with the treaty (curtsy of Flemeth and her words of wisdom - or riddles -) they were able to knock down the Blight before it truly had the chance to really start up.
*shrugs* With all the allies you had made, there wasn't really any reason to why the battle would be drawn out. And the fact they attacked Denerim indicated they knew where to strike (at heart of Thedas) so it was easy for the Wardens to strike a fatal blow.
(either that or the Archdemon had some sort of awareness thank to being 'awoken' and had a death wish).