What takes longer than people expect when creating a 3D design?

People usually assume the hardest part is building the model, but I've found that isn't always the case. Getting the proportions right, finding suitable materials, sourcing furniture, making sure everything fits together, and handling small changes can take up far more time than expected. Sometimes the basic model comes together pretty quickly, but getting all the details to work together is where most of the effort goes. For those working in 3D or architecture, which part of the process tends to take you the longest?