When should you order tickets for an intercontinental flight when the return date is several months after the initial flight?

You'll only know the optimal time to book after the fact. There are just too many factors at play.

Generally, for intercontinental flights, it is best to book at least 3 months prior to departure. Most airlines will allow you to book up to 11 or 12 months in advance. This means you can book as early as right now or as late as end of May.

In general it is best to start by gauging typical prices for the route. This can be accomplished by doing repeated search for trips on various dates (including those that wont work for you) to get a feel for what the cheapest possible fare is going to be.

Once you've got a feeling for the price range you should start to monitor the dates you actually can use. If you find a price near the bottom end of the range, book it.

If you have the option of not traveling, you can wait and hope for a good last minute fare or otherwise hold out for 'just the right price'. However, if you have to travel and especially if you are traveling on a holiday, do not wait too long.

The good news is that the two main Hong Kong holiday periods are over (Lunar new year and Easter which is just finishing). They can be the most expensive times to fly.

Prices will rise again around your return trip before Christmas. You may want to try and go back earlier if you want to reduce the price.

Check out the public holidays in Hong Kong for an indication of when traveling would be most expensive:

http://www.gov.hk/en/about/abouthk/holiday/

In addition, the mainland China 'golden week' holidays will increase the price. The next one begins 1st October.

Finally, if you are really price conscious, then consider flying instead to Shenzen or Guangzhou airports and take the bus into HK.