Firstly relax, you have over a year to finish it! I failed Art GCSE so honestly, not the best person to advise how long art should take, but that's to highlight you have plenty of time to figure it out if you don't just yet
For Architecture my friend did a lot of technical drawings and studies of statues, memorials, buildings (at least a couple so you show it's not just all flashy stuff) and so on (not sure if you're familiar with Cardiff, but he was asked about his analysis of the War Memorial he sketched and researched). Generally, you just need to do that, plus analyse them like that
He also added some art pieces in various different styles; not sure if that's specifically required, but it does help if you have a particular talent. The portfolio is just there to be a more visual aid about why you're interested in Architecture compared to just the statement c:
If you're going to do that though I'd suggest finding structures that actually interest you a bit, because you'll likely be asked about them in the interview (if your choices have one).
Good luck!