No, that doesn't sound like an overly long time.
You say that "surely it can't take this long"? Why not? Whilst it's possible to come up with great stuff quickly, music is a craft and it takes grafting away at and work. Even if you can
improvise amazing game music, think of all the countless hours, experiences etc it took to get to the stage where that is possible.
It's like cooking. Eating it might not take very long, but the time that went into preparing it was much more extensive and lengthy. And it's worth it. And unlike a meal you can keep listening to it over and over again. (ok, you don't listen to meals anyway but ... shut up! My metaphor broke down, okay?

)
The answer to your question is, anywhere between the length of the track itself (basically improvised), to ... well, you could work on it for a potentially infinite amount of time.
Including many other responsibilities and a few days off, I just basically finished around 10 tracks for a game in a little over a week. I think that's relatively quick, but it would have been possible for me to do it much faster / slower depending on necessity. I have to say though that I am a believer in doing things without spending years trying to perfect them, because all the experience you learn in the years you spend writing other stuff will mean when you come back to the stuff you wrote earlier you will know how to improve it anyway. I don't really agree with endless perfectionism personally, but whatever works for others is whatever works for them!