WOW! How Long do modeling these usually take?
..Not the Mashup new design for the Egg....
Long Answer: For myself, I started for the first time with any modeling tool about last August '09. My only background is computer science and math and can't draw to save my life. It took a couple of months of going through the Akira tutorial that used to be on this forum and danmangames to get comfortable with xsi. The main thing is learning how to take primitive shapes and manipulate them into something that matches what you want. Having orthographic views of the design can also help when building the model. Personally, I've found following drawing techniques has helped me to create models I'm happy with. With sins the model doesn't have to have a lot of detail to look good. Here's an example of a low poly model I made and a high poly model I used to burn a bump map with and the end result.
http://dl.dropbox.com/u/5790092/SinsAlbum/light_frigate_wip.jpg (drawing technique)
http://dl.dropbox.com/u/5790092/SinsAlbum/light_frigate_low.jpg (low poly for in game)
http://dl.dropbox.com/u/5790092/SinsAlbum/light_frigate_high.jpg (high poly for bump map)
http://i951.photobucket.com/albums/ad356/ZombiesRus5/FrigateCyborgLight3.png (low poly with bump map applied).
I'm still weak when it comes to texturing and still have a lot to learn with modeling in general, but I still can create some decent stuff for in the game with what I know now.
http://dl.dropbox.com/u/5790092/Sins%20of%20the%20Fallen%20Album/lrf_nephilim_missile.jpg (recent model that turned out ok textured)
Short Answer: It depends on the quality and complexity of the model I guess. I usually spend a couple days building the low poly model for sins to the point I'm comfortable with the looks (maybe 2-12 hours depending on how quick it comes). Then the process of uv-mapping, texturing, maybe building a high poly for the bump map. You can invest a little bit of time in the process to generate a good mesh as the end result. I would give it a try if your interested and can get past the initial learning stage. It's good fun creating 3d models and seeing them in the game