Proof that game developers shouldn't rush sequels.
I've had the sequel to my Proximity game in the works for over a year now, after yet another year of sitting on the idea and letting it percolate in my mind on what could make the game a better experience. I have actually waited too long, as people are starting to forget the original game.
However, making (and releasing!) a sequel only a month after the original obviously is too short of a time to wait. This game suffered from fuzzy graphics, making things more complicated for the sake of being more complication than actually adding a better game experience, making things even more reliant on chance, and a modified GUI that obstructs part of the playing field. Letting this game sit for a little while and taking a look at it every day without actually doing anything to it before releasing it would have revealed that these are obvious problems interfering with people's enjoyment of the game.
I highly suggest to the game author to have more than one series in the works, so that he can alternate between them (and ocassionally add a new series) before working on the sequels to one of his earlier games, that way his mind can work subconsciously on the problem of what needs to be done to improve the game instead of just immediately latching on to whatever suggestion sounded appealling in the reviews left on the previous game.
The demand for the game will still be there after the wait, and you'll still get a good amount of sponsorship money (if that's what you're worried about) once you actually get around to making the game. In fact, you might even get more money for the game, and you don't risk ruining the reputation the series has made in the process.
Two years can be too long in the web game world (although not necessarily). Nine months is probably a good time to wait, though, and get it out after a year. Just imagine, if you put out a sequel to this game year after year, every year, the game would end up having about 20 more sequels than even the most prolific commercial game series get. You don't need to rush yourself that much.