Earlier this year, when I was working on the Colosseum, I spent about 2 months only to get about 1/3 of the model done, but now I would say i'm about 85% after about 2 weeks!
The reason this model takes so long is because of all the intricate measurements required to make sure everything alligns properly and the model doesn't get out of proportion due to it's Elliptical shape. Also bare in mind that I have to make objects so that they can be textured proportionately aswell...this is not half life 2 where you can select a face and scale a tile texture on it. This is an actual in game object made up of a whole bunch of little objects, all with their own UV texture maps.
I COULD just leave the texture maps until some other time, but when it comes to running the map in game, the surface of the object flickers crazy because of lighting issues (the game doesn't know how to project shadows and lighting on a surface with a funky UV map...or no UV map at all!).
One more reason that the model takes so long is the amount of time it takes to improvise; Alot of resource material on the Colosseum is conflicting, and it's difficult to tell which is right and which is wrong. I have tried to rely on photo resources and it has helped with some details that otherwise wouldn't be seen. It has also helped me to try and understand scale and layout, however due to the colosseum being in RUINS...it's kinda difficult to gather photo information for certain sections/parts.
After everything is done, I will have to do minor tweaks to make sure that everything is alligned properly and that there are no open seams (gaps that you can see through...gaps which light could come through and cause strange lighting effects).
I estimate that the actual model will be finished within 2 weeks TOPS (If I get distracted or something).
Then, I can get to work on textures (I am foreseeing something horribly messy!).
Finally after the texturing, time to get to work on even bigger landmarks...such as the Circus Maximus; However I won't be doing the interior for that one, i've had enough of modelling interiors of round shaped buildings.
So that's that. I will provide in engine screenshots when it's all ready. (I am not going to show an untextured, unfinished model unless it's a dud model like the screenshots I have on my profile).