The website doesn't tell you that crucial detail. Doh.
Point of information - in between the delights of Cheltenham station car park being closed and Swindon's impending improvement works, the rolling News banner on the home page does in fact tell you that Advance tickets are currently available until Tuesday May 26. 'Course, you have to i) spot this, and ii) realise that it also means that bike reservations etc are not yet available from May 27 on.
East Coast used to have the decency to put the date in a really obvious splash on the r/h side of the home page: now that Vermin has taken over, it's been moved to bottom left, where it's a lot easier to miss, especially if you've not maximised your browser window. At least it's still there and a static part of the page, unlike FGW.
More generally, the booking horizon is generally about 12 weeks: if you're trying to book tickets about that far ahead, the giveaway on booking sites is whether or not any Advance prices are quoted for your specific dates. I normally have a quick look at a random date a couple of months away, just to see what range of ticket types and prices I should be getting offered.