In your blog post on 10/19/2015 (Conference Meetings with an agent who has a ms on submission), you said it can take 6 months or more to answer a full. Your blog shows a six week backlog of queries. So seven and a half, perhaps eight months before an answer?
My goal is to have a non-fiction book published by the time of the 2017 BEA in anticipation of the Feb 2018 Winter Olympics. (It includes the biggest story of the 2002 Winter Olympics). Right now I have the full book proposal with two thousand words towards sample chapters.
1. Do I query widely? [I know the answer to this one!]
2. Query now and write like hell towards the full ms?
3. Concentrate on querying junior agents who may provide quicker turn-around?
4. Kiss off my goal?
I'm selling books for the Fall 2017 season right now, so you should be querying RIGHT NOW.
The good news is turn around time on proposals is significantly less than it is for novels.
The bad news: writing a good proposal takes a good long time.
Make SURE you've read Susan Rabiner's book Thinking Like Your Editor. It's the best book out there on non-fiction.
The other thing to remember is that you might think that the 2018 Winter Olympics is the best time to have this book on shelves, but people buy books about the Olympics regardless of whether it's an Olympic year or not.
Off the top of my head: The Boys in The Boat was pubbed in 2013. Sacred Games by my client Gary Corby was pubbed in 2013 as well. The Amateurs by David Halberstam was pubbed in 1985.
So worry more about writing a GREAT proposal and worry less about timing.