I’ve just been asked this question and for flights, I know that the answer is up to one year but I’ve checked and rail bookings have a surprisingly short selling period:
For Eurostar journeys to mainline Paris, Lille and Brussels stations, you can book up to 4 months in advance, minus 2 days
This applies to both French Rail, the UK arm of French SNCF railways and Eurostar. For the genuine links to both companies and for booking tips please visit my Eurostar Page
The best fares are booked in advance, however;
London to Paris, Brussels or Lille is only bookable up to 4 months (120 days) in advance.
EXAMPLE: Today 30th March we can only book for an outbound date of 28th July
To onward destinations, for example, Bordeaux, you can only book 3 months (90 days) in advance.
EXAMPLE: Today 30th March we can only book for an outbound date of 28th June.
This limit applies to the outbound AND return date so if you are staying for a week, you risk losing the cheapest seats on the outbound whilst you wait for the return to become available.
A travel agent should be able to reserve seats the outward whilst waiting for the return so if you are travelling peak time it may be worth paying an agent’s booking fee to get the lowest prices.
Make a diary note and book here!