I wanted to share my experiences finding partner award space on Star Alliance. The following generally applies to United Mileage Plus but it applies to all partner access (i.e. any Star Alliance partner accessing space with another Star Alliance airline). I did not take into account fuel surcharges. Each frequent flyer program determines if they charge fuel surcharges. If they do, it's important to consider which airline to choose as some fuel surcharges can get very expensive.
Bear in mind that these are just my observations – please chime in and share your successful (or not) experiences in the comments. I restrict myself to Business/First Class availability on long haul flights (6+ hrs).
Lufthansa
Business Class availability is being released 330 days out with daily changes. First Class comes into play 14 days before departure only. However many routes especially with the new First Class rarely ever open any First Class space. In general it seems at least 4 First Class seats must be free to have one opened up by the algorithm.
Usually as newer the aircraft as less likely it is that anything becomes available.
TAM
TAM is stingy with long haul award space. There are sprinklings of business class seats, but hardly anyone has every found First Class award space.
Austrian Airlines
I had no success finding Austrian Airlines business class availability ever. I know others have had success on some routes. I tried several on united.com and could not find anything.But that may be my limited searching?
Swiss Airlines
I have had luck with 330 days out to book Swiss Business Class. In general as further book as better availability seems to be. There is no additional set of seats being released in the last 14 days before departure. I have had my last First Class flight in 2011 with Swiss and have not seen any First Class seats being released to partners since then.
Turkish Airlines
There are a lot of Business Class Seats released to routes to Asia. Often at least open seat every day. Also flights to Houston are easily available. Turkish Airlines product is very different depending on what aircraft configuration you are getting so check before you fly.
TAP
There is generally good availability throughout the next 330 days for TAP flights from Europe to South America. Some routes go through Oporto instead of Lisbon, so make sure you check these as well.
Asiana
Award seat availability is very unpredictable with Asiana opening up many seats in both First Class and Business Class early on but also adding seats later. If your dates are flexible you'll always find a seat on Asiana (at least I did so far).
Singapore Airlines
Singapore Airlines does not release seats in Business/ First Class to partners usually. The exception are the routes on the old configuration business class on the 777-200 where it does without a predictable pattern. Routes include flights to Istanbul, Auckland, Perth.
ANA
The airlines does release seats especially in business class from early on. Additional seats become available in the last 48 hours before departure.
Air New Zealand
Business Class to the United States is not being released to partners. However routes to Asia often show good availability with routes like NRT showing seats every day.
Egyptair
Egyptair does release a number of seats on its metal for partner booking. In fact I often see Egyptair as the only available airline is business class many times. However it's not a very aspiring product.
LOT
I'm no expert on LOT but I have no luck finding any seats so far. I tried a number of times but haven't seen much business class availability. Please share your thoughts!
Thai Airlines
Thai makes a lot of seats available in Business and First Class. For many days every (!) single flight to Europe has seats available in business class! A380 First Class seems more restricted but even there seats exist in advance.
United
United does generally open seats up 330 days out and keeps adding seats sometimes. While the product is not stellar – it's one of my favorite airlines to fly with if I want to sleep – both the business and first class seats make for a great rest.
Ethiopian
From the routes I checked barely any business class was open for award booking – just Economy seats. But I'm not expert on the airline – maybe once the 787 is back in service things will improve?
South African
There is no First Class on South African but the good business class product shows availability year round on many routes. I have not found a specific predictable algorithm but don't count on seats being released last minute.
Scandinavian Airlines
Scandinavian Airlines does release business class seats – but it's really few in between. There might be one seat per route per month!
Air China
Air China has some good equipment in the sky like the 777 to LAX. Business and First Class availability is generally pretty good with many seats being released early on.
Air Canada
Air Canada does release business class seats (it has no First Class) but it's very spotty – one seat per month per route is no exception.
Brussels Airlines
Brussels Airlines is a small airline with few routes but their JFK route sees quite a number of seats being released – for those of you on the East Coast this may be a hidden gem.
How to find availability?
Find out which hubs an airline operates from. Then make a web search for the routes the airline operates. Finally plug in the correct route as one-way into united.com award search engine and see what happens for some dates. Use the 'nonstop-only' option to only show direct flights. Look for days on the calendar indicated in green and blue.
My conclusion
Given the reach of Star Alliance it's very easy to find a partner with Business Class availability for almost any destination if you have a flexible schedule.
First Class availability is much harder but Thai/Asiana/United and Lufthansa to a certain extent still allow you access to a very luxurious product with miles.
Still it's downright amazing how far a couple of credit card applications can take you with Star Alliance!