Depends on the build. Generally speaking most of my builds can be anywhere from a few hours to a day, depending on reference material, size and scaling (if scaling is needed to fit the structure within the height limit), and whether or not interiors of the structures are taken into account. If it is only exterior, the time can hover anywhere from 4 hours to 12. If interior is required, the time to complete will skyrocket.
I can't say for sure since I don't know what the possible builds are, but if some of the structures I have built are anything to go by, 5 days to complete the competition should be plenty (minus interior).
