How soon can you buy tickets for f1 2008 races?

[Saabyfox]

Have not been to an F1 race for years, so thought i would give Le Mans a miss this year and go to Spa instead.
How soon do they release the tickets for the races? They are not open for sale on F1.com?

[belleair302]

NOW.most circuits are selling grandstand seats. GPtickets.com