Just to add to Jeff's great post if I may? He's correct in saying there is no set time. The default for most programs and users is 2 seconds in between songs. That doesn't mean you have to use it though. :)
Like Jeff says, there are times when a song that has a fade, would have a smaller gap between the next song. A song that stops dead, would be better with a 2 second gap into the next song.
That said, there are 4 things to keep in mind here that are extremely important in my opinion.
1. You will make different decisions on how long or short your gaps will be if you do this procedure using headphones.
*In headphones, you hear each fade-out in full because they are right in your brain. This will not be the case in real monitors. This can cause you to make gaps longer while making this decision with headphones on.*
2. You will make different decisions on how long or short your gaps will be if you do this procedure with monitors.
*In real monitors, even at loud volumes, you will not hear the entire fade the way you will in headphones. So if you made the gap decision listening on headphones and processed it, when you listen through real monitors or car speakers, the gaps may appear too long when in reality, your fade out are still going on...you just can't hear them.*
**It is best to find a happy medium for 1 and 2. Don't rely on one listening environment to help you make the final decision. Use both headphones and monitors to find that happy gap medium.
3. You will make different decisions on how long or short your gaps will be if you do this procedure after listening to the material at a loud volume.
*When you listen to music loud for 30 minutes, you lose high end frequencies as well as some sensitivity. Make sure you take a break for about 2 hours before you proceed with any finalizing.*
4. Gaps that are too short between songs can cause what we call "album listener fatigue". Though this is nothing super serious, when gaps are too short one song sort of runs into the next and sometimes a nice little break for tension in an album is a good thing.
Gaps that are too long can make an album slow moving and boring. When you have a song that has a long fade-out, the gap for the next song should be shorter at around 1 second or 1.5 seconds. Depending on the length of the fade, you may even need a half a second or somewhere in between a half a second and 1 second.
As for your CD Architect question, you should be able to click on the song and slide it in increments away from the song before it. You have a time-line there and each song should have markers on it. When you slide the song, the markers move with it. It's been a long time since I've used that program, but that's how I remember it to be. Hope this helps...best of luck. :)
-Danny