I believe that to get it at the 9000 avios price, Eurostar need to be selling the ticket for £34.50 each way (so the return is the cheapest available at £69).

For a popular train, you'd therefore basically need to book the moment the tickets become available, which is around 120 days. For a less popular train (eg Wednesday mid afternoon), you might be in luck 3 weeks out.

The simplest thing is probably to search on the Eurostar website, look for the cheapest tickets on the rough days you want to travel, then ring up Avios to book. (In theory, you ought to be able to use the ITA Matrix to search, as I believe that Eurostar expose their trains via some travel agent systems, but I've never managed to get it to work when searching from QQS to XPG)