When combined with the design space it provides to enthusiastic GMs, this can lead to a campaign that stretches numerous real-time years. I think it's incredibly rare, however, to find a group that will stay engaged with a single campaign over 3+ years. I think its even rarer to find a GM who can keep up her or his energy up for that long.
Moreover, in my long experience behind the screen, I've always enjoyed ending a campaign with the players wanting more instead of lingering in malaise that eventually ends the campaign and possibly breaks up the gaming group.
To that end, unless you and your group are one of the rare exceptions I mentioned above, I recommend speeding things up if you're at 2+ years and are nowhere close to the end.
1. Ditch the emphasis on exploration. After Vordakai, I don't think my players spent more than one or two sessions exploring. One player was a little bummed by this, but I think he appreciated the increased pace.
4. Cut. I took out Irovetti's tournament altogether. At that point, the players were incredibly suspicious of him and it felt cheap to launch a surprise attack they would have suspected the entire time.
I'm with you on this. I am prepping for session 30, playing monthly (almost), and while no one's lost interest, I am in fear of it happening. Our last two APs deleted the last 2+ books. As such, I am squashing books 3, 4, & 5 together, so that we can get into book 6 with momentum behind us. The war is nearly upon them, and they know it will happen.