Really i don't think i had much of a needed clean up period as i like to pay attention to detail, and if a problem came up, i would try to fix it there and then. Most of my fixing up was just adding more of the forest assets in sparse areas, removing a few trees to get sunlight to shine on the right parts. Since i manually placed in the repeated forest assets, i had to make sure that most of them weren't sticking out the ground and they were not all facing the same way or that too many were the same size.
Lightmapping was a pain, especially if i wanted add extra parts to one asset which i did do, meaning readjusting the lightmap channel and giving parts less space. So mainly my clean up and checking issues were UDK common problems. Otherwise my package was organised properly and reimporting textures or assets then rebuilding lighting was a doddle. Build time on my computer at home for my finished level is around just over 30 minutes while building in the university labs was less than 20 minutes.
As shown below, my two packages are organised well. It makes finding and adjusting things really easy.