I look after the Topic website and put together a FAQ page on the site which addresses some of the issues raised here.
Generally, Headline acts are booked 9-12 months ahead, sometimes less, sometimes more... Dana and Susan Robinson, from the US, who are due November 9th, have been booked since August last year. All headline acts up to the end of 2006 were booked by mid-May.
As the Topic could fill its schedule 2-3 times over, headliners are usually not considered again before about 18-24 months... using Dana and Susan Robinson as an example again, when they come this autumn it will be 21 months since their last appearance. There are exceptions - Roger Sutcliffe, a Topic worthy for 40 years (and former club organiser) has an annual berth, and The Durbervilles did two end-of-year parties on the trot.
Double Header acts are booked more like 6 months in advance. Double Headers are a relatively new idea for the Topic: two acts take equal billing and split the door take. The standard is similar to Headline act quality and is one way of "making 4 more Mondays" in the year, offering a bit of flexibility, and giving newer and/or more esoteric acts a stage.
Then there are Featured Support slots (for a minimal contribution to expenses, paid out of the raffle). These are also good for newer acts, or more established but not full-time pros looking for more exposure and the chance to sell CDs, or people on tour from overseas with a Thursday-shaped gap in their schedules, or people going solo from bands, or whatever. These are booked a few weeks in advance; all the slots from September this year onwards are open.
And then there are floor singers, unbooked... there are still a couple of slots even on guest nights when a Featured Support act is booked, and a few more when there is not. And finally there is still time for Singers' and Musicians' nights once a month (which could, at a pinch, be hijacked if Paul Simon wanted to play at short notice). At a recent one of these several members of Bandersnatch and a couple of Mulberry Blue showed up, and at another the duo FYRISH came along; all later got Featured Support slots.
Taking a look back over the last 12 months (on our Past Gigs page, which goes back to 1970 or so), I see the Topic had 31 Headline guest nights, 10 Double Header acts (5 double header nights, obviously), 19 Featured Support acts - a total of 60 acts paid something or other. And as there were still 14 Singers' and Musicians' nights (incl all August) in that period, those 60 acts slotted into 38 weeks.
People can work their way up the slots: Tom Napper Tom Bliss did a featured support slot before being booked for a headline gig, for instance, as did Julie Ellison. And others. (You can do it the other way round too: Darren Poyzer did a headline act one year and came back the next to be featured support to Roy Bailey).
Anyway, feel free to have a look at the site for the FAQ, the forthcoming acts, the past gigs list and anything else - like the acts for the Topic's 50th anniversary season this autumn.