I want to give CouchSurfing a shot as I'm going to be travelling to Japan and my #1 priority would be to socialize.
In what time frame should I be looking for potential hosts?
Here's my recommendation as a former host of 30+ people on Couchsurfing.
Book commercial accommodation. You never know if you find someone or not, so it's best to prebook something for the duration of your trip, and then cancel as late as possible once you have confirmations for a given night.
Both Booking.com and Hostelworld.com allow free cancellations for up to 24 hours before check-in. Be aware that, unlike a hotel, hosts can cancel your stay at any time, so nothing is fully guaranteed.
Check how busy is the city which you plan to visit. NYC is going to be a lot more busy than Montpelier, Vermont.
Check if there are any special holidays around the days of your stay. I usually got as many as 20 requests for NYE in Prague, as compared to 1-2 requests for other days of the year.
Another thing to watch out for is the current season. A lot more people visit Europe in the summer as compared to winter, for example.
Depending on 1. and 2., send a message anytime between 30 and 5 days. More for big cities and holidays, less for small cities at a quiet time of the year.
Make you sure you state an exact date and an approximate time of arrival, as well as the exact number of people in your party. Try to message hosts who have logged into the website in the past 3 days, as this makes it much more likely that they will at least read your message.