Those Swiss freaks over at Counter-Strike.ch have posted an interview with the maker of Trinity and (more recently) Vegas. Here's a clip:
Check.[SIN]speed85: 2. When you make a map, like de_Vegas, how long do you need? Who helps you and with what does he helps you. What is your formula to produce a good map?
Cadaver: The map layout itself usually takes a couple of weeks to put together. But then you need to tweak it, so usually the time from start to completion is a month or so. I'm not reliant on anyone except my texture guy, George_Pooshoes, who helps me out by creating custom textures for my maps when needed. With vegas, I got some feedback from Gooseman about adding more cover and stuff, but that was about it; I got the most feedback from playing the map on public servers once b7 was out. I honestly don't really know what makes a good map - I enjoy vegas immensely, but judging from the CLQ ratings, the public doesn't. The most popular maps are "chokepoint" maps like assault, dust and prodigy, so I guess if you want your map to go down well, put in some chokepoints (ie places one team *has* to pass through to reach their objective).