Originally Posted by
findark
It's a little fuzzier with international fares, but pulling a random citypair (ORD-MUC), some of the lowest fares have 50-day AP requirements, although there are plenty of cheap fares with shorter ones too.
In general, until you get within about four weeks of departure you aren't significantly disadvantaged by purchasing late, unless it so happened that United earlier misjudged expected sales and accidentally offered a lower price (in lower inventory) in the past.
It's worth looking now just to get a sense, but I would just check every so often, including if you get a fare sale email from United. But for a September purchase you're likely okay waiting at least until May or June and perhaps longer, and prices aren't likely to go up massively right away.
Also check on events in the city you're headed to - for example I'd guess that an ORD-MUC ticket in mid-Sept. might be a bit more expensive because of Octoberfest.