For the retailer – one month is fine, or whenever all the single issues are sold out. It's so much easier to stock and track inventory on graphic novels (GNs) than for comics. GNs hold up better to fondling in-store, and have a higher profit margin. They're also just about the only format non-hardcore comics fans will realistically buy. GNs are basically the perfect format, with the exception of release cycle. Also, GNs are regularly stacked on shelves in real bookstores, at least the ones that still exist, which is something that can't be said for comics. Even Amazon will sell you a GN, but they pretty much won't touch individual issue sales.
For the publisher – A bit more variance, but the sooner the better. At this point, comics are a loss-leader for the GN market, and a canary in the coal mine that helps warn against continuing a storyline/arc/creative team should sales completely tank. As with retailers, GNs produce higher margins for the publisher… But also higher costs, so more than schedule, seeded demand is the key component. Basically, monthly comics are many times now laid out and paced with an eye for GN success, rather than packing more punch into the monthly – lots of two page spreads, and issues where nothing seems to happen. As well, storylines are paced and packaged and interwoven to help create cross-sell from a given title to others down the line. It doesn't always work, but series like Avengers Disassembled certainly helped sell both classic archival trades, as well as pre-sell the follow-on New Avengers continuing comic and GNs, which then split off into Mighty Avengers, Dark Avengers, Pet Avengers, etc etc etc. Oh yeah, and the sooner you get the GN out the door, the sooner the whole thing is "done", and you can move on to other projects.
For the reader – Let's face it, we want a GN or so per month, not 32 pages including ads. To be able to read a nice encapsulated story, to be able to pass it around to friends and not worry about one of them losing my issue of Dakota North #5 (fuck you, Chet!)… Er, I mean, and to be able to save between 30-50% of the retail cover price on a set of single issues telling the same story – well, that's just good all around. Various attempts at bonuses, the DVD extras of comicdom if you will, have largely been non-starters (just like real DVDs, go figure), but nevertheless there are opportunities to provide premiums and real value in a collected trade that fans can actually understand… Unlike, say, the "foil cover" version of Superman #75 – In which our hero dies, inevitably to be brought back to life after a one year hiatus, WITH A FOIL COVER WHICH MAKES IT SOOOOO MUCH BETTER!
Simply put: it's in everyone but the artist's interests to move as quickly as possible to create a graphic novel release, assuming there is a likelihood it will sell at least to break-even in the marketplace. If you look outside the US market, to places like Europe or Japan, the hardcore market is able to easily pay for and read up to 4 times the pagecount per month for titles they enjoy, but it's really the release of the GNs that spike sales and interest in a comic book property over the long haul. Discounting the artist's need for sleep/carpal healthcare, then, it's even in their interests, should they actually want to make a living of their dream job. 😉